anchor.diff:      if (_isNumber(secondOp, 0)) {
         return firstOp;
       }
  +    
  +    if (secondOp is UnaryMinus) {
  +      return firstOp - secondOp.exp; // a + -(b) = a - b
  +    }
   
       return new Plus(firstOp, secondOp);
       //TODO -a + b = b - a
  
positive.code: 
      if (_isNumber(secondOp, 0)) {
        return firstOp;
      }
  
      return new Minus(firstOp, secondOp);
  
negative.code: 
      if (_isNumber(firstOp, 0)) {
        return firstOp; // = 0
      }
  
      if (_isNumber(secondOp, 1)) {
  