class RDoc::Markdown

def link_to content, label = content, text = nil
  raise ParseError, 'enable notes extension' if
    content.start_with? '^' and label.equal? content
  if ref = @references[label] then
    "{#{content}}[#{ref}]"
  elsif label.equal? content then
    "[#{content}]#{text}"
  else
    "[#{content}]#{text}[#{label}]"
  end
end