exec (SeqStmt []) penv env = ("exn: No match in env2", H.empty, H.empty)
exec (SeqStmt (x:xs)) penv env = aux (x:xs) "" penv env
                                where aux [] x penv env = (x, penv, env)
                                      aux (s:ss) x penv env = aux ss (x++j) (H.union penv1 penv) (H.union env1 env)
                                        where (j, penv1, env1) = exec s penv env