if (key == null || nth == null | list == null)
throw ERT.badarg(key, nth_arg, list_arg);
while (!list.isNil()) {
EObject elm = list.head();
ETuple tup = elm.testTuple();
// test that it is a tuple of the right size
if (tup != null && tup.arity() >= nth.value) {
EObject val = tup.elm(nth.value);
if (val.equals(key)) { return ERT.TRUE; }