class Ronn::Index

def <<(path)
  raise ArgumentError, "local paths only" if path =~ /(https?|mailto):/
  return self if any? { |ref| ref.path == File.expand_path(path) }
  relative_path = relative_to_index(path)
  @references << \
    if path =~ /\.ronn?$/
      reference manual(path).reference_name, relative_path
    else
      reference File.basename(path), relative_path
    end
  self
end