code: def find_unit_clause(clauses, model):
      
      for clause in clauses:
          P, value = unit_clause_assign(clause, model)
          if P: return P, value
      return None, None
docs: Find a forced assignment if possible from a clause with only 1
      variable not bound in the model.
      >>> find_unit_clause([A|B|C, B|~C, ~A|~B], {A:True})
      (B, False)