Ein einfacher Prolog-Interpreter

query  :: [Clause] -> [Atom] -> [Substitution]
query cs [] = return M.empty
query cs (a : as) = do
    u1 <- single cs a
    u2 <- query cs $ map ( apply u1 ) as
    return $ u1 `times` u2

single :: [Clause] ->  Atom  -> [Substitution]
single cs a = do
    c <- cs 
    let c' = rename c
    u1 <- maybeToList $ unify a $ head c'
    u2 <- query cs $ map ( apply u1 ) $ body c'
    return $ u1 `times` u2