code: def _init_go_sources(self, go_sources_arg, go2obj_arg):
          
          gos_user = set(go_sources_arg)
          if 'children' in self.kws and self.kws['children']:
              gos_user |= get_leaf_children(gos_user, go2obj_arg)
          gos_godag = set(go2obj_arg)
          gos_source = gos_user.intersection(gos_godag)
          gos_missing = gos_user.difference(gos_godag)
          if not gos_missing:
              return gos_source
          sys.stdout.write("{N} GO IDs NOT FOUND IN GO DAG: {GOs}\n".format(
              N=len(gos_missing), GOs=" ".join([str(e) for e in gos_missing])))
          return gos_source
docs: Return GO sources which are present in GODag.