gistfile1.rb

class Module
  VALID=[:optional, :rest_arg]
  def is(value)
    raise ArgumentError, "value must be one of #{VALID.join(',')}" unless VALID.include? value
    [self,value]
  end
end

a = [Range, Fixnum.is :optional, String.is :rest_arg] #=> [Range, [Fixnum, :optional], [String, :rest_arg]]