Simulating error and accept in actions

The parsing actions of error and accept can be simulated in an action by use of macros YYACCEPT and YYERROR. The YYACCEPT macro causes yyparse() to return the value 0; YYERROR causes the parser to behave as if the current input symbol had been a syntax error; yyerror() is called, and error recovery takes place. These mechanisms can be used to simulate parsers with multiple end-markers or context sensitive syntax checking.