CS 2010 Lab 7 Instructor Plan

A tree-of-sym is either
- empty
- (make-tree sym tree-of-sym tree-of-sym)

• Implement count-syms : tree-of-sym -> num
• Define tree-of-X
• Implement sum-tree : tree-of-num -> num
• Abstract to fold-tree:
  fold-tree : (X Y Y -> Y) Y tree-of-X -> Y
• Implement tree-syms : tree-of-sym -> list-of-sym