func: function revoke(bytes32 _operation) external {
          uint ownerIndex = m_ownerIndex[uint(tx.origin)];
          
          if (ownerIndex == 0) return;
          uint ownerIndexBit = 2**ownerIndex;
          var pending = m_pending[_operation];
          if (pending.ownersDone & ownerIndexBit > 0) {
              pending.yetNeeded++;
              pending.ownersDone -= ownerIndexBit;
              Revoke(tx.origin, _operation);
          }
      }