l.PushUserData(struct{}{})
l.NewTable()
l.SetUserValue(-2)
l.NewTable()
lua.SetFunctions(l, []lua.RegistryFunction{
	{"__newindex", func(l *lua.State) int {
		name := lua.CheckString(l, 2)
		for _, step := range *steps {
			lua.ArgumentCheck(l, step != name, 2, fmt.Sprintf("step %q already exists", name))
		}
		lua.CheckType(l, 3, lua.TypeFunction)
		l.UserValue(1)
		l.Replace(1)
		l.RawSet(1)
		*steps = append(*steps, name)
		return 0
	}},
	{"__index", func(l *lua.State) int {
		l.UserValue(1)
		l.Replace(1)
		l.RawGet(1)
		return 1
	}},
}, 0)
l.SetMetaTable(-2)
l.SetGlobal("step")