function confirmAndCheck(bytes32 _operation) internal returns (bool) {
  // determine what index the present sender is:
  uint256 index = ownerIndex[msg.sender];
  // make sure they're an owner
  if (index == 0) return;

  var pending = pendings[_operation];
  // if we're not yet working on this operation, switch over and reset the confirmation status.
  if (pending.yetNeeded == 0) {
    // reset count of confirmations needed.
    pending.yetNeeded = required;
    // reset which owners have confirmed (none) - set our bitmap to 0.
    pending.ownersDone = 0;
    pending.index = pendingsIndex.length++;
    pendingsIndex[pending.index] = _operation;
  }
  // determine the bit to set for this owner.
  uint256 ownerIndexBit = 2**index;
  // make sure we (the message sender) haven't confirmed this operation previously.
  if (pending.ownersDone & ownerIndexBit == 0) {
    Confirmation(msg.sender, _operation);
    // ok - check if count is enough to go ahead.
    if (pending.yetNeeded <= 1) {
      // enough confirmations: reset and run interior.
      delete pendingsIndex[pendings[_operation].index];
      delete pendings[_operation];
      return true;
    } else {
      // not enough: record that this owner in particular confirmed.
      pending.yetNeeded--;
      pending.ownersDone |= ownerIndexBit;
    }
  }
}