def traverse(flag=nil,&op)
		op.call(self)
		all_children_deep(flag).each do |c|
			op.call(c)
		end
	end
def traverse(flag=nil,&op)
		op.call(self)
		all_children_deep(flag).each do |c|
			op.call(c)
		end
	end
Execute an operation on the node itself and all children,
 direct and indirect.