Version 0.3
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1/16/2018  Usability improvement on Lexer class.  Regular expression rules
           specified as strings that don't match any name in tokens are
           now reported as errors.

Version 0.2
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12/24/2017 The error(self, t) method of lexer objects now receives a
           token as input.  The value attribute of this token contains
           all remaining input text.  If the passed token is returned
           by error(), then it shows up in the token stream where
           can be processed by the parser.