code: func Join(sep cty.Value, lists ...cty.Value) (cty.Value, error) {
  	args := make([]cty.Value, len(lists)+1)
  	args[0] = sep
  	copy(args[1:], lists)
  	return JoinFunc.Call(args)
  }
docs: // Join concatenates together the string elements of one or more lists with a
  // given separator.