examples: bistromathic: demonstrate error recovery

* examples/c/bistromathic/parse.y: here.
* examples/c/bistromathic/bistromathic.test: Check it.
Included a stupid case where the error is actually ignored.

examples/c/bistromathic/README.md

@@ -2,6 +2,7 @@
 This example demonstrates best practices when using Bison.
 - Its hand-written scanner tracks locations.
 - Its interface is pure.
+- It uses the `error` token to get error recovery.
 - Its interface is "incremental", well suited for interaction: it uses the
   push-parser API to feed the parser with the incoming tokens.
 - It features an interactive command line with completion based on the

examples/c/bistromathic/bistromathic.test

@@ -82,11 +82,26 @@ run 0 '> 1 / 0
 > ''
 err: 1.1-5: error: division by zero'
 
+# Error recovery.
 cat >input <<EOF
-100%
+((1 ++ 2) ** 3)
+(1 ++ 2) + (3 ** 4)
 EOF
-run 0 '> 100%
-100
+run 0 '> ((1 ++ 2) ** 3)
+666
+> (1 ++ 2) + (3 ** 4)
+1332
+> ''
+err: 1.6: syntax error: expected - or ( or number or function or variable before +
+err: 2.5: syntax error: expected - or ( or number or function or variable before +
+err: 2.16: syntax error: expected - or ( or number or function or variable before *'
+
+# This is really stupid: we just discarded % and did not enter error recovery.
+cat >input <<EOF
+100% + 10
+EOF
+run 0 '> 100% + 10
+110
 > ''
 err: 1.4: error: invalid character'
 

examples/c/bistromathic/parse.y

@@ -146,6 +146,7 @@ exp:
 | "-" exp  %prec NEG { $$ = -$2; }
 | exp[l] "^" exp[r]  { $$ = pow ($l, $r); }
 | "(" exp ")"        { $$ = $2; }
+| "(" error ")"      { $$ = 666; }
 ;
 
 // End of grammar.