res = lambda a, b: [(a[i], b[(i+1)%2]) for i in [0,1] if a[(i+1)%2] == b[i]]
 
def isconsistent(inequations):
  pending, done, hashes = [inequations[:], [], set()]
  while pending:
    p = pending.pop()
    if p[0] == p[1]:
      return False
    for n in (n for d in done for n in res(p, d) if "%s<%s"%n not in hashes):
      hashes.add("%s<%s"%n)
      pending.append(n)
    done.append(p)
  return True
 
print(isconsistent([("A", "B"), ("B", "C"), ("C", "D")]))
print(isconsistent([("A", "B"), ("B", "C"), ("C", "A")]))

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