public String differentiate(Parser.Addition add){
                Expression leftExpr;
                Expression rightExpr;
                String outLeft;
                String outRight;
                leftExpr = add.left;
                rightExpr = add.right;
                outLeft = Differentiator.differentiate(leftExpr, variable);
                outRight = Differentiator.differentiate(rightExpr, variable);
                return "(" + outLeft + " " + "+" + " " + outRight+ ")";
            }