module Module::Concerning

def concern(topic, &module_definition)

end
...

extend ActiveSupport::Concern
module EventTracking

is equivalent to

end
...
concern :EventTracking do

A low-cruft shortcut to define a concern. 
def concern(topic, &module_definition)
  const_set topic, Module.new {
    extend ::ActiveSupport::Concern
    module_eval(&module_definition)
  }
end