Function process

exprtk_test.cpp:9871–9897

   using usr_t::process;

   bool process(const std::string& unknown_symbol,
                usr_symbol_type& st,
                T& default_value,
                std::string& error_message) exprtk_test_override
   {
      if (unknown_symbol[0] == 'v')
      {
         st = usr_t::e_usr_variable_type;
         default_value = next_value();
         error_message.clear();

         return true;
      }
      else if (unknown_symbol[0] == 'c')
      {
         st = usr_t::e_usr_constant_type;
         default_value = next_value();
         error_message.clear();

         return true;
      }
      else
      {
         error_message = "Unknown symbol...";
         return false;
      }
   }

   T next_value(const bool reset = false)
   {