Problem 4: Trictionary or Treat (200pts)

A trictionary is a pair of tree k and v. They have identical structure: each node in k has a corresponding node in v. The labels in k are called keys. Each key may be the label for multiple nodes in k, and the values for that key are the labels of all the corresponding nodes in v.

A lookup function returns one of the values for a key. Specifically, a lookup function for a node in k is a function that takes v as an argument and returns the label for the corresponding node in v.

Implement the generator function lookups, which takes as input a tree k and a key. It yields all lookup functions for nodes in k that have key as their label, the functions could be yielded in any order.

def lookups(k, key):
    """Yield one lookup function for each node of k that has the label key.
    >>> k = tree(5, [tree(7, [tree(2)]), tree(8, [tree(3), tree(4)]), tree(5, [tree(4), tree(2)])])
    >>> v = tree('Go', [tree('C', [tree('C')]), tree('A', [tree('S'), tree(6)]), tree('L', [tree(1), tree('A')])])
    >>> type(lookups(k, 4))
    <class 'generator'>
    >>> sorted([f(v) for f in lookups(k, 2)])
    ['A', 'C']
    >>> sorted([f(v) for f in lookups(k, 3)])
    ['S']
    >>> [f(v) for f in lookups(k, 6)]
    []
    """
    "*** YOUR CODE HERE ***"