code: def tt_entails(kb, alpha):
      
      assert not variables(alpha)
      return tt_check_all(kb, alpha, prop_symbols(kb & alpha), {})
docs: Does kb entail the sentence alpha? Use truth tables. For propositional
      kb's and sentences. [Fig. 7.10]
      >>> tt_entails(expr('P & Q'), expr('Q'))
      True