code: def v_init_extension(ctx, stmt):
      
      (prefix, identifier) = stmt.raw_keyword
      (modname, revision) = \
          prefix_to_modulename_and_revision(stmt.i_module, prefix,
                                            stmt.pos, ctx.errors)
      stmt.keyword = (modname, identifier)
      stmt.i_extension_modulename = modname
      stmt.i_extension_revision = revision
      stmt.i_extension = None
docs: find the modulename of the prefix, and set `stmt.keyword`