Defining subtrees by the node at the root is very useful, but in terms of the definition of a subtree, there can be multiple subtrees per root node. By subtree, we mean any subset of nodes and links from the original tree that combine to form a valid tree with the links still following one$-$directional parent$-$child relationships. With this definition in mind, how many subtrees are there of the following tree? $6-1-5-2$