docs: function ls_ (req, depth, cb) {
    if (typeof cb !== "function") cb = depth, depth = 1
    mkdir(npm.cache, function (er) {
      if (er) return log.er(cb, "no cache dir")(er)
      function dirFilter (f, type) {
        return type !== "dir" ||
          ( f && f !== npm.cache + "/" + req
           && f !== npm.cache + "/" + req + "/" )
      }
      find(path.join(npm.cache, req), dirFilter, depth, function (er, files) {
        if (er) return cb(er)
        return cb(null, files.map(function (f) {
          f = f.substr(npm.cache.length + 1)
          f = f.substr((f === req ? path.dirname(req) : req).length)
               .replace(/^\//, '')
          return f
        }))
      })
    })
  }