function collectPayment(uint256 _index) external returns (bool) {
    require(payments[_index].status == Status.Requested);
    require(payments[_index].requestedTime + 24*60*60 < block.timestamp);
    require(balances[debtor] >= payments[_index].amountWei);
    balances[debtor] = balances[debtor].sub(payments[_index].amountWei);
    balances[creditor] = balances[creditor].add(payments[_index].amountWei);
    payments[_index].status = Status.Paid;
    return true;
  }