(prog
  (fun f (lambda x (* x x)))
  (fun g (lambda a b (+ a b)))
  (var cat 5)
  (var dog true)
  (apply f 4)

)


        prog
      /      \
    fun       fun
   /  \       /  \
  f  lambda  g    lambda
     /   \        /  |  \
    x     *      a   b   +
         / \            / \
        x   x          a   b


Global Environment
[ f    [LAMBDA_TAG (lambda x (* x x))]   ]
[ g    [LAMBDA_TAG (lambda a b (+ a b))] ]
[ cat  [INT_TAG 5]                       ]
[ dog  [BOOL_TAG true]                   ]