I don't believe this is a valid return. The values should be written as defined statements and then the following should happen "return ((lower_op_four_bound<=string_to_int(operand.substr(1,operand.size())))&& (string_to_int(operand.substr(1,operand.size()))<=upper_op_four_bound))  . Although should probably just do a variable set prior to the return statement to avoid the lenght issue