def md_ref_def(ref_id, url, title=nil, meta={}, al=nil)
		meta[:url] = url
		meta[:ref_id] = ref_id
		meta[:title] = title if title
		md_el(:ref_definition, [], meta, al)
	end
def md_ref_def(ref_id, url, title=nil, meta={}, al=nil)
		meta[:url] = url
		meta[:ref_id] = ref_id
		meta[:title] = title if title
		md_el(:ref_definition, [], meta, al)
	end
[1]: http://url [properties]