token getter method. If you use 'do_parse' method, you must also
implement this method. The format of return value is
[TOKEN-SYMBOL, VALUE]. token-symbol is represented by Ruby's symbol
as default. For example, :IDENT for 'IDENT' in Racc. ";" (String) for ';'.
Last symbol (EOF) must be false.
called when parse error is found.
err_tok is an internal token number which makes error.
err_val is its value.
_values is a stack of symbol values. DO NOT EDIT this object.
This method raises ParseError as default.
If you override this method and
make not raise exceptions, parser enters into "error recovering" mode.