/** return a list of elements at given key */
@Override
protected ESeq lookup(EObject key) {
IPersistentMap ipm = deref();
IPersistentCollection set = (IPersistentCollection) ipm.valAt(key);
ESeq res = ERT.NIL;
if (set == null) return res;
for(ISeq s = set.seq(); s != null; s = s.next())
{
res = res.cons((EObject) s.first());