&gt;You really don&#x27;t know what it is?<p>What if I don&#x27;t? Some of us program more abstract things than the others. For example I might want to write a function, that given a tree, and, well, a value, returns a tree where all nodes that have this value are removed. Doesn&#x27;t matter that the tree in question will in practice be a tree of Flooblooators, I&#x27;d rather write reusable code that works for everything.