If a CNF contains only Horn and Xor clauses, then what is the complexity of determining Satisfiability? If a CNF contains only Horn and Xor clauses, and does not contain clauses of other types, then can its Satisfiability be determined in polynomial time?
Hint: Using XOR clauses you can express "$x = \lnot y$", and this allows you to simulate general clauses by Horn clauses.