Find all the GO descendants under all user GO IDs. Return leaf-level GO IDs.

def get_leaf_children(gos_user, go2obj_arg):
    """Find all the GO descendants under all user GO IDs. Return leaf-level GO IDs."""
    childgoid2obj = {}
    for goid_usr in gos_user:
        goobj_usr = go2obj_arg[goid_usr]
        fill_childgoid2obj(childgoid2obj, goobj_usr)
    return set(go for go, o in childgoid2obj.items() if not o.children)