code: public static String diffReadable(String expression) throws InvalidExpressionException {
  
          ExpressionParser p = new ExpressionParser();
  
          return p.format(diff(expression));
      }
docs: Compute the reformatted symbolic derivative.
  @param expression the mathematical expressino
  @return "readable" form of the derived expression.  Unnecessary * are deleted and
  unary minus is changed from $ to -.