anchor.diff:      rpnTokenStreams.add([new Token("x", TokenType.VAR),
                            new Token("2", TokenType.VAL),
                            new Token("+", TokenType.PLUS)]);
  -    
  +
       inputStrings.add("log(10,100)");
       tokenStreams.add([new Token("log", TokenType.LOG),
                       new Token("(", TokenType.LBRACE),
  
positive.code:                         'log(,1)': throwsRangeError,
                          'log(1,)': throwsRangeError,
      };
      
      for (String expr in invalidCases.keys) {
        expect(() => pars.parse(expr), invalidCases[expr]);
      }
  
negative.code:                          new Token("1", TokenType.VAL),
                           new Token("-", TokenType.MINUS),
                           new Token("nrt", TokenType.ROOT)]);
  
      inputStrings.add("1^1^1");
      tokenStreams.add([new Token("1", TokenType.VAL),
                        new Token("^", TokenType.POW),
  