Abbreviating routes when :format is presented

Hi you all,

I have some routes like the following, one for a possible HTML route,
and the others when a :format parameter is presented, like .xml:

  map.bugs_by "bugs/by/:att/:value", :controller => "bugs",
:action => "by",
:att => nil, :value =>nil

  map.connect "bugs/by.:format", :controller => "bugs", :action => "by"
  map.connect "bugs/by/:att.:format", :controller => "bugs", :action =>
  map.connect "bugs/by/:att/:value.:format", :controller => "bugs",
:action => "by"

Is there a simple way to define the routes when :format is presented and
the other parameters can be nil? Cause if not, routes.rb file can be a
bit large.