on_error(t, val, vstack)
public
This method is called when a parse error is found.
ERROR_TOKEN_ID is an internal ID of token which caused error. You can get string representation of this ID by calling #token_to_str.
ERROR_VALUE is a value of error token.
value_stack is a stack of symbol values. DO NOT MODIFY this object.
This method raises ParseError by default.
If this method returns, parsers enter “error recovering mode”.