Let the initial-action act on the look-ahead, and use it for the

"initial push" (corresponding to an hypothetical beginning-of-file).
And let lalr1.cc honor %initial-action.
* doc/bison.texinfo (Initial Action Decl): Clarify, and add an
example.
* data/lalr1.cc (Parser::initlocation_): Remove, bad experiment.
(Parser::Parser): Remove the ctor that used to initialize it.
(Parser::parse): Like in the other skeletons, issue the "starting
parse" message before any action.
Honor %initial-action.
Initialize the stacks with the lookahead.
* data/yacc.c: Let $$ and @$ in %initial-action designate the
look-ahead.
Push them in the stacks.
* tests/actions.at, tests/calc.at: Adjust the C++ ctor invocations.

data/lalr1.cc

@@ -218,17 +218,9 @@ namespace yy
     typedef Stack< SemanticType > SemanticStack;
     typedef Stack< LocationType > LocationStack;
 
-#if YYLSP_NEEDED
-    ]b4_parser_class_name[ (bool debug,
-	    LocationType initlocation][]b4_param[]b4_parse_param_decl[) :
-      ]b4_constructor[][debug_ (debug),
-      cdebug_ (std::cerr),
-      initlocation_ (initlocation)]b4_parse_param_cons[
-#else
     ]b4_parser_class_name[ (bool debug][]b4_param[]b4_parse_param_decl[) :
       ]b4_constructor[][debug_ (debug),
       cdebug_ (std::cerr)]b4_parse_param_cons[
-#endif
     {
     }
 
@@ -325,18 +317,17 @@ namespace yy
     /* Message.  */
     std::string message;
 
-    /* Semantic value and location of look-ahead token.  */
+    /// Semantic value of the look-ahead.
     SemanticType value;
+    /// Location of the look-ahead.
     LocationType location;
     /// The locations where the error started and ended.
     Location error_range_[2];
 
-    /* @@$ and $$.  */
+    /// $$.
     SemanticType yyval;
+    /// @@$.
     LocationType yyloc;
-
-    /* Initial location.  */
-    LocationType initlocation_;
 ]b4_parse_param_vars[
   };
 }
@@ -446,24 +437,34 @@ yy::]b4_parser_class_name[::pop (unsigned int n)
 int
 yy::]b4_parser_class_name[::parse ()
 {
+  YYCDEBUG << "Starting parse" << std::endl;
+
   nerrs_ = 0;
   errstatus_ = 0;
 
-  /* Initialize the stacks.  The initial state will be pushed in
-     yynewstate, since the latter expects the semantical and the
-     location values to have been already stored, initialize these
-     stacks with a primary value.  */
-  state_stack_ = StateStack (0);
-  semantic_stack_ = SemanticStack (1);
-  location_stack_ = LocationStack (1);
-
   /* Start.  */
   state_ = 0;
   looka_ = empty_;
-#if YYLSP_NEEDED
-  location = initlocation_;
-#endif
-  YYCDEBUG << "Starting parse" << std::endl;
+
+]m4_ifdef([b4_initial_action], [
+m4_pushdef([b4_at_dollar],     [location])dnl
+m4_pushdef([b4_dollar_dollar], [value])dnl
+  /* User initialization code. */
+  b4_initial_action
+m4_popdef([b4_dollar_dollar])dnl
+m4_popdef([b4_at_dollar])dnl
+/* Line __line__ of yacc.c.  */
+b4_syncline([@oline@], [@ofile@])])dnl
+
+[  /* Initialize the stacks.  The initial state will be pushed in
+     yynewstate, since the latter expects the semantical and the
+     location values to have been already stored, initialize these
+     stacks with a primary value.  */
+  state_stack_ = StateStack (0);
+  semantic_stack_ = SemanticStack (0);
+  location_stack_ = LocationStack (0);
+  semantic_stack_.push (value);
+  location_stack_.push (location);
 
   /* New state.  */
 yynewstate:

data/yacc.c

@@ -821,20 +821,23 @@ b4_c_function_def([yyparse], [int], b4_parse_param)
 ]b4_location_if([[  yylsp = yyls;
 #if YYLTYPE_IS_TRIVIAL
   /* Initialize the default location before parsing starts.  */
-  yyls[0].first_line   = yyls[0].last_line   = 1;
-  yyls[0].first_column = yyls[0].last_column = 0;
+  yylloc.first_line   = yylloc.last_line   = 1;
+  yylloc.first_column = yylloc.last_column = 0;
 #endif
 ]])
 m4_ifdef([b4_initial_action], [
-m4_pushdef([b4_at_dollar],     [(*yylsp)])dnl
-m4_pushdef([b4_dollar_dollar], [(*yyvsp)])dnl
+m4_pushdef([b4_at_dollar],     [(yylloc)])dnl
+m4_pushdef([b4_dollar_dollar], [(yylval)])dnl
   /* User initialization code. */
   b4_initial_action
 m4_popdef([b4_dollar_dollar])dnl
 m4_popdef([b4_at_dollar])dnl
 /* Line __line__ of yacc.c.  */
 b4_syncline([@oline@], [@ofile@])])dnl
-
+[
+  yyvsp[0] = yylval;
+]b4_location_if([[    yylsp[0] = yylloc;
+]])
 [  goto yysetstate;
 
 /*------------------------------------------------------------.