def valid_fallback?(fallback)
      return true if fallback.nil? && allow_nil?
      return true if include? fallback

      false
    end
def valid_fallback?(fallback)
      return true if fallback.nil? && allow_nil?
      return true if include? fallback

      false
    end
Check if the provided `fallback` is valid

 @param [String, Symbol, nil] fallback