Define a procedural representation of a binary tree. A binary tree is either a terminal node that contains a value or a nonterminal node containing two children that are both binary trees. Define two constructors, leaf and interior, for creating terminal and nonterminal trees. Define an observer, traverse, that traverses the tree in a depth-first manner, applies op to each value stored in the tree, and applies combine to combine values from the root node, left subtree, and right subtree to produce a single value: