def clear_merged! r=false
            @merged.each do |p|
                p.clear_merged! r
                r.release_p p if r
            end
            @merged.clear
        end
def clear_merged! r=false
            @merged.each do |p|
                p.clear_merged! r
                r.release_p p if r
            end
            @merged.clear
        end
recursively clears the merged Particle list

 @param [Boolean] r releases the cleared Particle if true