code: function getPayment(account, identifier, callback) {
    var self = this;
  
    validate.address(account);
    validate.paymentIdentifier(identifier);
  
    // If the transaction was not in the outgoing_transactions db,
    // get it from rippled
    function getTransaction(_callback) {
      transactions.getTransaction(self, account, identifier, {}, _callback);
    }
  
    var steps = [
      getTransaction,
      asyncify(_.partial(formatPaymentHelper, account))
    ];
  
    async.waterfall(steps, callback);
  }
docs: Retrieve the details of a particular payment from the Remote or
  the local database and return it in the ripple-rest Payment format.
  
  @param {Remote} remote
  @param {/lib/db-interface} dbinterface
  @param {RippleAddress} req.params.account
  @param {Hex-encoded String|ASCII printable character String}
  req.params.identifier