class Rack::Session::Abstract::ID

def self.inherited(klass)
  k = klass.ancestors.find { |kl| kl.respond_to?(:superclass) && kl.superclass == ID }
  unless k.instance_variable_defined?(:"@_rack_warned")
    warn "#{klass} is inheriting from #{ID}.  Inheriting from #{ID} is deprecated, please inherit from #{Persisted} instead" if $VERBOSE
    k.instance_variable_set(:"@_rack_warned", true)
  end
  super
end