def clear
      if @sticky.empty? then
        @ckh.clear
      else
        @ckh.each { |klass, ch| ch.clear unless @sticky.include?(klass) }
      end
    end
def clear
      if @sticky.empty? then
        @ckh.clear
      else
        @ckh.each { |klass, ch| ch.clear unless @sticky.include?(klass) }
      end
    end
Clears the non-sticky class caches.