anchor.diff:      }
   
       if (_isNumber(exponentOp, 0)) {
  -      return new Number(1); // x^0 = 1
  +      return new Number(1.0); // x^0 = 1
       }
   
       if (_isNumber(exponentOp, 1)) {
  
positive.code:    */
    Variable(String this.name);
  
    Expression derive(String toVar) => name == toVar ? new Number(1) : new Number(0);
  
    String toString() => '$name';
  
  
negative.code:     throw new UnsupportedError('Number $this can not be interpreted as: ${type}');
    }
  
    Expression derive(String toVar) => new Number(0.0);
  }
  
  /**
  