mirror of
https://git.savannah.gnu.org/git/bison.git
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doc: catch up with the current display of cex
Unfortunately I found no way to use the ↳ glyph in Texinfo, so I used
@arrow{} instead, which has a different width, so we have to have all
the examples doubled, once for TeX, another for the rest of the world.
* doc/bison.texi: Use the current display in the examples.
* doc/calc.y, doc/ids.y, doc/if-then-else.y, doc/sequence.y: New.
This commit is contained in:
Akim Demaille
276
doc/bison.texi
276
doc/bison.texi
@@ -9889,10 +9889,11 @@ initially developed by Chinawat Isradisaikul and Andrew Myers
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As a first example, see the example grammar of @ref{Shift/Reduce}, which
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features on shift/reduce conflict:
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@c see doc/if-then-else.y
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@example
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$ @kbd{bison if-then-else.y}
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if-then-else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
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if-then-else.y: warning: rerun with option '-Wcounterexamples' to generate conflict counterexamples [-Wother]
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if-then-else.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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if-then-else.y: @dnotice{note}: rerun with option '-Wcounterexamples' to generate conflict counterexamples
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@end example
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@noindent
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@@ -9900,32 +9901,46 @@ Let's rerun @command{bison} with the option
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@option{-Wcex}/@option{-Wcounterexamples}@inlinefmt{info, (the following
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output is actually in color)}:
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@ifhtml
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@ifnottex
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@example
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Shift/reduce conflict on token "else":
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if-then-else.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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if-then-else.y: @dwarning{warning}: shift/reduce conflict on token "else" [@dwarning{-Wcounterexamples}]
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@group
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Example @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @blue{"else" stmt}
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Shift derivation @yellow{if_stmt @arrow{} [ "if" expr "then"} @green{stmt @arrow{} [} @blue{if_stmt @arrow{} [ "if" expr "then" stmt} @red{•} @blue{"else" stmt ]} @green{]} @yellow{]}
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Example @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @yellow{"else" stmt}
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Reduce derivation @yellow{if_stmt @arrow{} [ "if" expr "then"} @green{stmt @arrow{} [} @blue{if_stmt @arrow{} [ "if" expr "then" stmt} @red{•} @blue{]} @green{]} @yellow{"else" stmt ]}
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Example: @yellow{"if" expr "then"} "if" expr "then" stmt • "else" stmt
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Shift derivation
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@yellow{if_stmt}
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@yellow{↳ "if" expr "then"} @green{stmt}
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@green{↳} @blue{if_stmt}
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@blue{↳ "if" expr "then" stmt} @red{•} @blue{"else" stmt}
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Example: @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @yellow{"else" stmt}
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Reduce derivation
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@yellow{if_stmt}
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@yellow{↳ "if" expr "then"} @green{stmt} @yellow{"else" stmt}
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@green{↳} @blue{if_stmt}
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@blue{↳ "if" expr "then" stmt} @red{•}
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@end group
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@end example
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@end ifhtml
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@ifnothtml
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@smallexample
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Shift/reduce conflict on token "else":
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@end ifnottex
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@iftex
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@example
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if-then-else.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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if-then-else.y: @dwarning{warning}: shift/reduce conflict on token "else" [@dwarning{-Wcounterexamples}]
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@group
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Example
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@yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @blue{"else" stmt}
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Example: @yellow{"if" expr "then"} "if" expr "then" stmt • "else" stmt
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Shift derivation
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@yellow{if_stmt @arrow{} [ "if" expr "then"} @green{stmt @arrow{} [} @blue{if_stmt @arrow{} [ "if" expr "then" stmt} @red{•} @blue{"else" stmt ]} @green{]} @yellow{]}
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Example
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@yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @yellow{"else" stmt}
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@yellow{if_stmt}
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@yellow{@arrow{} "if" expr "then"} @green{stmt}
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@green{@arrow{}} @blue{if_stmt}
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@blue{@arrow{} "if" expr "then" stmt} @red{•} @blue{"else" stmt}
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Example: @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @yellow{"else" stmt}
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Reduce derivation
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@yellow{if_stmt @arrow{} [ "if" expr "then"} @green{stmt @arrow{} [} @blue{if_stmt @arrow{} [ "if" expr "then" stmt} @red{•} @blue{]} @green{]} @yellow{"else" stmt ]}
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@yellow{if_stmt}
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@yellow{@arrow{} "if" expr "then"} @green{stmt} @yellow{"else" stmt}
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@green{@arrow{}} @blue{if_stmt}
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@blue{@arrow{} "if" expr "then" stmt} @red{•}
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@end group
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@end smallexample
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@end ifnothtml
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@end example
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@end iftex
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This shows two different derivations for one single expression. That
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demonstrates that the grammar is ambiguous.
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@@ -9935,6 +9950,7 @@ demonstrates that the grammar is ambiguous.
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As a more delicate example, consider the example grammar of
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@ref{Reduce/Reduce}, which features a reduce/reduce conflict:
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@c doc/sequence.y
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@example
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%%
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sequence:
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@@ -9950,32 +9966,112 @@ maybeword:
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Bison generates the following counterexamples:
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@ifnottex
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@example
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@group
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$ @kbd{bison -Wcex sequence.y}
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sequence.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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sequence.y: @dwarning{warning}: 2 reduce/reduce conflicts [@dwarning{-Wconflicts-rr}]
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Shift/reduce conflict on token "word":
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Example @red{•} @green{"word"}
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Shift derivation @yellow{sequence @arrow{} [} @green{maybeword @arrow{} [} @red{•} @green{"word" ]} @yellow{]}
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Example @red{•} @yellow{"word"}
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Reduce derivation @yellow{sequence @arrow{} [} @green{sequence @arrow{} [} @red{•} @green{]} @yellow{"word" ]}
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Reduce/reduce conflict on tokens $end, "word":
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Example @red{•}
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First derivation @yellow{sequence @arrow{} [} @red{•} @yellow{]}
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Example @red{•}
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Second derivation @yellow{sequence @arrow{} [} @green{maybeword @arrow{} [} @red{•} @green{]} @yellow{]}
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Shift/reduce conflict on token "word":
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Example @red{•} @green{"word"}
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Shift derivation @yellow{sequence @arrow{} [} @green{maybeword @arrow{} [} @red{•} @green{"word" ]} @yellow{]}
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Example @red{•} @yellow{"word"}
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Reduce derivation @yellow{sequence @arrow{} [} @green{sequence @arrow{} [} @blue{maybeword @arrow{} [} @red{•} @blue{]} @green{]} @yellow{"word" ]}
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@end group
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@group
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sequence.y: @dwarning{warning}: shift/reduce conflict on token "word" [@dwarning{-Wcounterexamples}]
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Example: @red{•} @green{"word"}
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Shift derivation
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@yellow{sequence}
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@yellow{↳} @green{maybeword}
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@green{↳} @red{•} @green{"word"}
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Example: @red{•} @yellow{"word"}
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Reduce derivation
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@yellow{sequence}
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@yellow{↳} @green{sequence} @yellow{"word"}
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@green{↳} @red{•}
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@end group
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@group
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sequence.y: @dwarning{warning}: reduce/reduce conflict on tokens $end, "word" [@dwarning{-Wcounterexamples}]
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Example: @red{•}
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First reduce derivation
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@yellow{sequence}
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@yellow{↳} @red{•}
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Example: @red{•}
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Second reduce derivation
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@yellow{sequence}
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@yellow{↳} @green{maybeword}
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@green{↳} @red{•}
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@end group
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@group
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sequence.y: @dwarning{warning}: shift/reduce conflict on token "word" [@dwarning{-Wcounterexamples}]
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Example: @red{•} @green{"word"}
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Shift derivation
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@yellow{sequence}
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@yellow{↳} @green{maybeword}
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@green{↳} @red{•} @green{"word"}
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Example: @red{•} @yellow{"word"}
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Reduce derivation
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@yellow{sequence}
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@yellow{↳} @green{sequence} @yellow{"word"}
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@green{↳} @blue{maybeword}
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@blue{↳} @red{•}
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@end group
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@group
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sequence.y:8.3-45: @dwarning{warning}: rule useless in parser due to conflicts [@dwarning{-Wother}]
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8 | @dwarning{%empty @{ printf ("empty maybeword\n"); @}}
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| @dwarning{^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}
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@end group
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@end example
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@end ifnottex
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@iftex
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@example
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@group
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$ @kbd{bison -Wcex sequence.y}
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sequence.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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sequence.y: @dwarning{warning}: 2 reduce/reduce conflicts [@dwarning{-Wconflicts-rr}]
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@end group
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@group
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sequence.y: @dwarning{warning}: shift/reduce conflict on token "word" [@dwarning{-Wcounterexamples}]
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Example: @red{•} @green{"word"}
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Shift derivation
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@yellow{sequence}
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@yellow{@arrow{}} @green{maybeword}
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@green{@arrow{}} @red{•} @green{"word"}
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Example: @red{•} @yellow{"word"}
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Reduce derivation
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@yellow{sequence}
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@yellow{@arrow{}} @green{sequence} @yellow{"word"}
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@green{@arrow{}} @red{•}
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@end group
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@group
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sequence.y: @dwarning{warning}: reduce/reduce conflict on tokens $end, "word" [@dwarning{-Wcounterexamples}]
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Example: @red{•}
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First reduce derivation
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@yellow{sequence}
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@yellow{@arrow{}} @red{•}
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Example: @red{•}
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Second reduce derivation
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@yellow{sequence}
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@yellow{@arrow{}} @green{maybeword}
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@green{@arrow{}} @red{•}
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@end group
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@group
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sequence.y: @dwarning{warning}: shift/reduce conflict on token "word" [@dwarning{-Wcounterexamples}]
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Example: @red{•} @green{"word"}
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Shift derivation
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@yellow{sequence}
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@yellow{@arrow{}} @green{maybeword}
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@green{@arrow{}} @red{•} @green{"word"}
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Example: @red{•} @yellow{"word"}
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Reduce derivation
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@yellow{sequence}
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@yellow{@arrow{}} @green{sequence} @yellow{"word"}
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@green{@arrow{}} @blue{maybeword}
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@blue{@arrow{}} @red{•}
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@end group
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@group
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sequence.y:8.3-45: @dwarning{warning}: rule useless in parser due to conflicts [@dwarning{-Wother}]
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8 | @dwarning{%empty @{ printf ("empty maybeword\n"); @}}
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| @dwarning{^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}
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@end group
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@end example
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@end iftex
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Each of these three conflicts, again, prove that the grammar is ambiguous.
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For instance, the second conflict (the reduce/reduce one) shows that the
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@@ -9989,6 +10085,7 @@ that are the same up until the dot. Most notably, this will happen when
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your grammar requires a stronger parser (more lookahead, LR instead of
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LALR). The following example isn't LR(1):
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@c doc/ids.y
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@example
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%token ID
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%%
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@@ -9999,13 +10096,58 @@ expr: %empty | expr ID ','
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@command{bison} reports:
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@smallexample
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Shift/reduce conflict on token ID:
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First example @purple{expr} @red{•} @purple{ID ','} @green{ID} @yellow{$end}
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Shift derivation @yellow{$accept @arrow{} [} @green{s @arrow{} [} @blue{a @arrow{} [} @purple{expr @arrow{} [ expr} @red{•} @purple{ID ',' ]} @blue{]} @green{ID ]} @yellow{$end ]}
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Second example @blue{expr} @red{•} @green{ID} @yellow{$end}
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Reduce derivation @yellow{$accept @arrow{} [} @green{s @arrow{} [} @blue{a @arrow{} [ expr} @red{•} @blue{]} @green{ID ]} @yellow{$end ]}
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@end smallexample
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@ifnottex
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@example
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ids.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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ids.y: @dwarning{warning}: shift/reduce conflict on token ID [@dwarning{-Wcounterexamples}]
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@group
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First example: @purple{expr} @red{•} @purple{ID ','} @green{ID} @yellow{$end}
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Shift derivation
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@yellow{$accept}
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@yellow{↳} @green{s} @yellow{$end}
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@green{↳} @blue{a} @green{ID}
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@blue{↳} @purple{expr}
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@purple{↳ expr} @red{•} @purple{ID ','}
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Second example: @blue{expr} @red{•} @green{ID} @yellow{$end}
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Reduce derivation
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@yellow{$accept}
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@yellow{↳} @green{s} @yellow{$end}
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@green{↳} @blue{a} @green{ID}
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@blue{↳ expr} @red{•}
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@end group
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@group
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ids.y:4.4-7: @dwarning{warning}: rule useless in parser due to conflicts [@dwarning{-Wother}]
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4 | a: expr
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| ^~~~
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@end group
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@end example
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@end ifnottex
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@iftex
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@example
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ids.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-sr}]
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ids.y: @dwarning{warning}: shift/reduce conflict on token ID [@dwarning{-Wcounterexamples}]
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@group
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First example: @purple{expr} @red{•} @purple{ID ','} @green{ID} @yellow{$end}
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Shift derivation
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@yellow{$accept}
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@yellow{@arrow{}} @green{s} @yellow{$end}
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@green{@arrow{}} @blue{a} @green{ID}
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@blue{@arrow{}} @purple{expr}
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@purple{@arrow{} expr} @red{•} @purple{ID ','}
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Second example: @blue{expr} @red{•} @green{ID} @yellow{$end}
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Reduce derivation
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@yellow{$accept}
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@yellow{@arrow{}} @green{s} @yellow{$end}
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@green{@arrow{}} @blue{a} @green{ID}
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@blue{@arrow{} expr} @red{•}
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@end group
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@group
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ids.y:4.4-7: @dwarning{warning}: rule useless in parser due to conflicts [@dwarning{-Wother}]
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4 | a: expr
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| ^~~~
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@end group
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@end example
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@end iftex
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This conflict is caused by the parser not having enough information to know
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the difference between these two examples. The parser would need an
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@@ -10041,6 +10183,7 @@ by default. As a consequence, the verbose output file is called
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The following grammar file, @file{calc.y}, will be used in the sequel:
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@c doc/calc.y
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@example
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@group
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%union
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@@ -10084,7 +10227,7 @@ calc.y:19.1-7: @dwarning{warning}: nonterminal useless in grammar: useless [@dwa
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19 | @dwarning{useless: STR;}
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| @dwarning{^~~~~~~}
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calc.y: @dwarning{warning}: 7 shift/reduce conflicts [@dwarning{-Wconflicts-sr}]
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calc.y: @dwarning{warning}: rerun with option '-Wcounterexamples' to generate conflict counterexamples [@dwarning{-Wother}]
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calc.y: @dnotice{note}: rerun with option '-Wcounterexamples' to generate conflict counterexamples
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@end smallexample
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Going back to the calc example, when given @option{--report=state},
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@@ -10396,15 +10539,44 @@ When given @option{--report=counterexamples}, @command{bison} will generate
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counterexamples within the report, augmented with the corresponding items
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(@pxref{Counterexamples}).
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@ifnottex
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@example
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Shift/reduce conflict on token '/':
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shift/reduce conflict on token '/':
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1 exp: exp '+' exp •
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4 exp: exp • '/' exp
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Example @yellow{exp '+'} @green{exp} @red{•} @green{'/' exp}
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Shift derivation @yellow{exp @arrow{} [ exp '+'} @green{exp @arrow{} [ exp} @red{•} @green{'/' exp ]} @yellow{]}
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Example @green{exp '+' exp} @red{•} @yellow{'/' exp}
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Reduce derivation @yellow{exp @arrow{} [} @green{exp @arrow{} [ exp '+' exp} @red{•} @green{]} @yellow{'/' exp ]}
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@group
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Example: exp '+' exp • '/' exp
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Shift derivation
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exp
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↳ exp '+' exp
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↳ exp • '/' exp
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Example: exp '+' exp • '/' exp
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Reduce derivation
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exp
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↳ exp '/' exp
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↳ exp '+' exp •
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@end group
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@end example
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@end ifnottex
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@iftex
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@example
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shift/reduce conflict on token '/':
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1 exp: exp '+' exp •
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4 exp: exp • '/' exp
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@group
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Example: exp '+' exp • '/' exp
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Shift derivation
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exp
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@arrow{} exp '+' exp
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@arrow{} exp • '/' exp
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Example: exp '+' exp • '/' exp
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Reduce derivation
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exp
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@arrow{} exp '/' exp
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@arrow{} exp '+' exp •
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@end group
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@end example
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@end iftex
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This shows two separate derivations in the grammar for the same @code{exp}:
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@samp{e1 + e2 / e3}. The derivations show how your rules would parse the
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@@ -15854,7 +16026,7 @@ London, Department of Computer Science, TR-00-12 (December 2000).
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@c LocalWords: TokenKind Automake's rtti Wcounterexamples Chinawat PLDI
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@c LocalWords: Isradisaikul tcite pcite rgbGreen colorGreen rgbYellow Wcex
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@c LocalWords: colorYellow rgbRed colorRed rgbBlue colorBlue rgbPurple
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@c LocalWords: colorPurple ifhtml ifnothtml situ rcex
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@c LocalWords: colorPurple ifhtml ifnothtml situ rcex MERCHANTABILITY Wnone
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@c Local Variables:
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@c ispell-dictionary: "american"
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