The Probabilistic Tree Creator tries to create symbolic expression trees with uniformly distributed length.

Initial phase: create a root node and a start node (which together amount for an initial tree length of 1), and begin expanding until the target length is reached. This works by initially placing dummy extension points onto the available children placeholders, then replacing them with subtrees of specific depth and length.

Next phase: pick random extension points, replace them with subtrees (of a minimal or otherwise constrained size), depending on the current depth and length of the tree.