function requestPayment(uint256 _index, uint256 _rateEthJpy) external onlyCreditor returns (bool) {
    require(payments[_index].status == Status.Pending || payments[_index].status == Status.Rejected);
    require(payments[_index].paymentDue <= block.timestamp);
    payments[_index].rateEthJpy = _rateEthJpy;
    payments[_index].amountWei = payments[_index].amountJpy.mul(ethWei).div(_rateEthJpy);
    payments[_index].requestedTime = block.timestamp;
    payments[_index].status = Status.Requested;
    return true;
  }