code: public SharedTreeSubgraph getSharedTreeSubgraph(final int tidx, final int cls) {
      if (tidx < 0 || tidx >= _output._ntrees) {
        throw new IllegalArgumentException("Invalid tree index: " + tidx +
                ". Tree index must be in range [0, " + (_output._ntrees -1) + "].");
      }
      final CompressedTree auxCompressedTree = _output._treeKeysAux[tidx][cls].get();
      return _output._treeKeys[tidx][cls].get().toSharedTreeSubgraph(auxCompressedTree, _output._names, _output._domains);
    }
docs: Converts a given tree of the ensemble to a user-understandable representation.
  @param tidx tree index
  @param cls tree class
  @return instance of SharedTreeSubgraph