Once a possibility is applied, the new subtree will be substituted into the
parent node of the subtree. This requires a subtree_map to avoid traversing the
expression tree. The subtree can also be the root node of the expression tree.
In that case, there is no parent node found for the subtree. Otherwise the old
subtree will be substituted with the new subtree node in the parent node.
Also updated the test cases accordingly.