Issues with following transpositions in Interactive Lessons

[Siderite]

When you reach the end of a line that continues with a transposition and you have sequential and/or spaced repetition enabled, the transposition is not followed.

[Site "https://lichess.org/study/1z3mukVH/NaJlmOGF"]
1. d4 d5 2. Nc3 c5 (2... Nf6 3. Bf4 c5 4. e3 e6) 3. Bf4 Nf6

Culprit is this line:

        const nextMoves = allNextMoves
          .filter(c => !(this.options.flow.sequential || this.options.flow.spacedRepetition) || this.inCurrentPath(c.path));

There are several issues here:

• with the current implementation, without flow settings, completing a line that ends in a transposition allows you to jump to another branch. If you complete it, then both of them are marked completed, even if you did not follow the original line
  • here there should be a system to only mark a line once in a single run
• with flow settings, you should be able to jump to another branch to complete the line, but somehow freely explore.
  • here, maybe inCurrentPath should always return true after marking a line true, but make sure to check this also in other places.

[tvthompsonii]

I think you are going to have to conceptualize this as if you copy the tree from where you transposed to and graft it to the transposition location. For example, say you traverse the move tree to node A and then transpose to node B. You would want ot copy the tree starting at B and graft it after node A. There are two reasons for this:

1. You can't mark the final node X as done since it could be reached by two different paths.
2. You can't mark the node A complete since there could be branching in the tree starting at B.
   For this reason you need all endpoints of all paths represented as unique nodes in the tree. When that is the case you can mark a path complete when it reaches any endpoint.
   If you have this "complete tree" then you can traverse it depth first and record all the end-to-end paths. These are the pathways that you can randomly choose between when testing the player and mark complete when successfully completed.

[Siderite]

Problem is that the "path" of a node is already a Lichess concept, which doesn't consider transpositions. A node has a single path which represents each move that precedes the node in the tree of moves, not how you actually got there. With transpositions you may also have cycles, which makes this rather complex.

For now, transpositions are basically ignored in sequential/spaced repetition modes. Will have to think of a solution, but at the time I have no time.