code: function expand( day ) {
    switch ( day ) {
    case 'weekend':
    case 'weekday':
      return _( days ).pickBy( value => value.type === day ).keys();
    default:
      return [ day ];
    }
  }
docs: ## Expand Special Keys Given 'weekend' or 'weekday', this function will return a list of the relevant enum keys. If a regular key is provided, pass it through.