from itertools import *
def f(a, b, c, d):
    return ((a and b) == (not(c))) and (b <= d)
table = [(1, 0, 0, 0), (1, 0, 1, 0), (1, 0, 1, 1), (1, 1, 0, 0)]
for p in permutations('abcd'):
    if [f(**dict(zip(p, r))) for r in table] == [1, 1, 1, 1]:
        print(p)