#!/usr/bin/env python3
def UnitProp(S):
  Q = [ c for c in S if len(c)==1 ]
  while len(Q)>0:
    z = Q.pop()[0]; T = []
    for C in S:
      C = [j for j in C if j!=-z]
      if len(C)==0: return False
      if len(C)==1: Q.append(C)
      if not z in C: T.append(C)
    if len(T)==0: return True
    S = T
  return S

if __name__ == "__main__":
  assert UnitProp([ [1], [-1,2] ,[-1,3,4] ]) == [ [3,4] ]