code: def _force_edge_active_move(self, state: _STATE) -> _STATE:
          
          seqs, edges = state
          unused_edges = edges.copy()
  
          # List edges which do not belong to any linear sequence.
          for seq in seqs:
              for i in range(1, len(seq)):
                  unused_edges.remove(self._normalize_edge((seq[i - 1], seq[i])))
  
          edge = self._choose_random_edge(unused_edges)
          if not edge:
              return seqs, edges
  
          return (
              self._force_edge_active(seqs,
                                      edge,
                                      lambda: bool(self._rand.randint(2))),
              edges)
docs: Move which forces a random edge to appear on some sequence.
  
          This move chooses random edge from the edges which do not belong to any
          sequence and modifies state in such a way, that this chosen edge
          appears on some sequence of the search state.
  
          Args:
            state: Search state, not mutated.
  
          Returns:
            New search state with one of the unused edges appearing in some
            sequence.