def each_element(e_node_type=nil, &block) 
		@children.each do |c| 
			if c.kind_of? MDElement
				if (not e_node_type) || (e_node_type == c.node_type)
					block.call c
				end
				c.each_element(e_node_type, &block)
			end
		end
	end
def each_element(e_node_type=nil, &block) 
		@children.each do |c| 
			if c.kind_of? MDElement
				if (not e_node_type) || (e_node_type == c.node_type)
					block.call c
				end
				c.each_element(e_node_type, &block)
			end
		end
	end
Yields to each element of specified node_type
 All elements if e_node_type is nil.