def set_default(fallback)
      if valid_fallback?(fallback) || fallback == NO_DEFAULT
        @default = fallback
      else
        raise InvalidFallback, "Cannot set #{fallback.inspect} as default", caller
      end
    end
def set_default(fallback)
      if valid_fallback?(fallback) || fallback == NO_DEFAULT
        @default = fallback
      else
        raise InvalidFallback, "Cannot set #{fallback.inspect} as default", caller
      end
    end
Set the default fallback value.

 @param [String, Symbol, nil] fallback
 @return [void]