class Racc::Parser

If this method returns, parsers enter "error recovering mode".

This method raises ParseError by default.

DO NOT MODIFY this object.
value_stack is a stack of symbol values.

ERROR_VALUE is a value of error token.

#token_to_str.
You can get string representation of this ID by calling
ERROR_TOKEN_ID is an internal ID of token which caused error.

This method is called when a parse error is found.

def on_error(t, val, vstack)
  raise ParseError, sprintf("parse error on value %s (%s)",
                            val.inspect, token_to_str(t) || '?')
end