anchor.diff:      rpnTokenStreams.add([new Token("0", TokenType.VAL),
                            new Token("1", TokenType.VAL),
                            new Token("-", TokenType.MINUS)]);
  -    
  +
       inputStrings.add("(0 - 1)");
       tokenStreams.add([new Token("(", TokenType.LBRACE),
                         new Token("0", TokenType.VAL),
  
positive.code:                       new Token(")", TokenType.RBRACE)]);
      rpnTokenStreams.add([new Token("1", TokenType.VAL),
                           new Token("_", TokenType.UNMINUS)]);
    }
  
    /*
  
negative.code:       // also test for expression content.
      }
    }
  
    void parserExpressionTestInvalid() {
      Map invalidCases = {
                          '': throwsArgumentError,
  