code: func (l *State) IsString(index int) bool {
  	if _, ok := l.indexToValue(index).(string); ok {
  		return true
  	}
  	_, ok := l.indexToValue(index).(float64)
  	return ok
  }
docs: // IsString verifies that the value at index is a string, or a number (which
  // is always convertible to a string).
  //
  // http://www.lua.org/manual/5.2/manual.html#lua_isstring