def md_ref_def(ref_id, url, title=nil, meta={}, al=nil)
      all_meta = meta.merge({ :url => url, :ref_id => ref_id })
      all_meta[:title] ||= title
      md_el(:ref_definition, [], all_meta, al)
    end
def md_ref_def(ref_id, url, title=nil, meta={}, al=nil)
      all_meta = meta.merge({ :url => url, :ref_id => ref_id })
      all_meta[:title] ||= title
      md_el(:ref_definition, [], all_meta, al)
    end
A definition of a reference (e.g. `[1]: http://url [properties]`).