public String differentiate(Parser.Multiplication mul){
                Expression leftExpr;
                Expression rightExpr;
                String outLeft;
                String outRight;
                leftExpr = mul.left;
                rightExpr = mul.right;
                String rightDiff = Differentiator.differentiate(rightExpr, variable);
                String leftDiff = Differentiator.differentiate(leftExpr, variable);
                outLeft = "(" + leftExpr.toString() + " " + "*" + " " + rightDiff + ")";
                outRight = "(" + rightExpr.toString() + " " + "*" + " " + leftDiff + ")";
                return "(" + outLeft + " " + "+" + " " + outRight + ")";
            }