gistfile1.txt

var Builtins = []*Function{
  ...
  {
		Name: "first",
		Fast: Len,
		Func: initializeBuiltins("first"),
    ...
  }
  ...
}

func initializeBuiltins(name string) func(args ...any) (any, error) {
   return func(args ...any) (any, error) {
      Builtins[Index["first"]].Func = realFirstFunc
      Builtins[Index["last"]].Func = realLastFunc
      ...
      return Builtins[Index[name]](args...)
   }
}