contract PullPayment {

  using SafeMath for uint256;

  mapping(address => uint256) public payments;
  uint256 public totalPayments;

  /**
   * @dev withdraw accumulated balance, called by payee.
   */
  function withdrawPayments() public {
    address payee = msg.sender;
    uint256 payment = payments[payee];

    require(payment != 0);
    require(this.balance >= payment);

    totalPayments = totalPayments.sub(payment);
    payments[payee] = 0;

    assert(payee.send(payment));
  }

  /**
   * @dev Called by the payer to store the sent amount as credit to be pulled.
   * @param dest The destination address of the funds.
   * @param amount The amount to transfer.
   */
  function asyncSend(address dest, uint256 amount) internal {
    payments[dest] = payments[dest].add(amount);
    totalPayments = totalPayments.add(amount);
  }

}
/**
 * @title PullPayment
 * @dev Base contract supporting async send for pull payments. Inherit from this
 * contract and use asyncSend instead of send.
 */
function withdrawPayments() public {
    address payee = msg.sender;
    uint256 payment = payments[payee];

    require(payment != 0);
    require(this.balance >= payment);

    totalPayments = totalPayments.sub(payment);
    payments[payee] = 0;

    assert(payee.send(payment));
  }
/**
 * @dev withdraw accumulated balance, called by payee.
 */