HOMEWROK 7.

1. Write three binary predicates pre_order, in_order, and post_order that for a given 
   binary tree return a list that corresponds respectively pre_order, in_order, and
   post_order traversal of the tree.

   Pre-order, in-order, and post-order traversal visit each node in a tree by recursively 
   visiting each node in the left and right subtrees of the root. If the root node 
   is visited before its subtrees, this is preorder; if after, postorder; if between, 
   in-order.