func: function recharge(string code) public isActive callFromHuman(msg.sender) payable {
          require(msg.value >= minRecharge, "not enough fund");
  
          string memory _code = coinHolders.getRefCode(msg.sender);
          if (bytes(_code).length > 0) {
              code = _code;
          } else {
              if (!dealers.dealerExisted(code)) {
                  code = defaultRefCode;
              }
              coinHolders.setRefCode(msg.sender, code);
          }
  
          coinHolders.add(msg.sender, msg.value);
          roundIn(msg.value, code);
      }