Recuperación de Errores

[~/srcPLgrado/jison/jison-aSb(error)]$ cat aSb.jison 
%lex
%%
\s+             {}
[ab]            { return yytext; }
.               { return "INVALID"; }
/lex
%%
S: /* empty */  { $$ = ''; console.log("empty"); }
   | 'a' S 'b'  { $$ = $1 + $2 + $3; console.log("S -> aSb"); }
   | 'a' S error
;
%%

        parse: function parse(input) {
              var self = this,
                  stack = [0],
                  vstack = [null], // semantic value stack
                  lstack = [], // location stack
                  ...
                  recovering = 0,
                  TERROR = 2,
                  EOF = 1;
            while (true) {
                  // retreive state number from top of stack
                  state = stack[stack.length - 1];
  
                  ...
  
                  // handle parse error
>>                _handle_error: if (typeof action === 'undefined' || !action.length || !  ...action[0]) {
  
                      var errStr = '';
                      if (!recovering) {