fn pub file_url(directory: ref Path, path: ref Path) -> String {
  let rel = path.strip_prefix(directory).or_else(fn { path.clone })

  if rel.to_string == INDEX_FILE {
    '/'
  } else if rel.tail == INDEX_FILE {
    '/${rel.directory.with_extension('')}/'
  } else {
    '/${rel.with_extension('')}/'
  }
}