"""
cons(a, b) constructs a pair, and car(pair) and cdr(pair) return the first
and last element of that pair.

    car(cons(3, 4)) == 3
    cdr(cons(3, 4)) == 4

Given this implementation of cons, implement car and cdr.
"""


def cons(a, b):
    return lambda f: f(a, b)


def car(pair):
    return pair(lambda a, b: a)


def cdr(pair):
    return pair(lambda a, b: b)


# --- tests ---

assert car(cons(3, 4)) == 3
assert cdr(cons(3, 4)) == 4
assert car(cons("a", "b")) == "a"
assert cdr(cons("a", "b")) == "b"

print("All tests passed!")