docs: def watch_pending_transactions(self, callback):
          '''
          Callback will receive one argument: the transaction object just observed
  
          This is equivalent to `eth.filter('pending')`
          '''
          self.pending_tx_watchers.append(callback)
          if len(self.pending_tx_watchers) == 1:
              eth.filter('pending').watch(self._new_pending_tx)