code: def get_leaf_children(gos_user, go2obj_arg):
      
      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)
docs: Find all the GO descendants under all user GO IDs. Return leaf-level GO IDs.