docs: function install_ (pkg, reg, seen, mustHave, pkglist, bundles, cb) {
    log.verbose(pkg, "install_")
    if (seen[pkg]) return cb() // repeat, skip it
    seen[pkg] = true
  
    // it's a local thing or a url if it has a / in it.
    if (pkg.indexOf("/") !== -1 || pkg === ".") {
      log.silly(pkg, "install local")
      return cache.add(pkg, finisher(pkg, reg, pkglist, bundles, cb))
    }
  
    // now we know it's not a URL or file,
    // so handle it like a tag, version, or range.
    pkg = pkg.split("@")
    var name = pkg[0]
      , defTag = npm.config.get("tag")
      , ver = pkg.slice(1).join("@").trim() || defTag
      , range = semver.validRange(ver)
      , exact = semver.valid(ver)
      , tag = !exact && !range && range !== "" && ver
    log.verbose([pkg, mustHave], "must install?")
    pkg = pkg.join("@")
    seen[name+"@"+ver] = true
  
    // if there is a satisfying version already, then simply move on.
    if (!tag && findSatisfying(pkg, name, range, mustHave, reg)) {
      return cb()
    }
  
    // at this point, assume that it has to be installed.
    if (exact) {
      log.verbose("exact", pkg)
      // just pull the data out of the cache to ensure it's there
      return cache.read(name, exact, finisher(pkg, reg, pkglist, bundles, cb))
    }
  
    getData(name, function (er, data) {
      if (er) return cb(er)
      log.silly(data, pkg)
      if (tag) {
        log.verbose(tag, pkg+" tag")
        var tags = data["dist-tags"]
        if (!tags[tag]) cb(new Error(
          "Tag not found: "+data.name+"@"+tag
          +"\nValid install targets for "+data.name+": "
          +installTargets(data)))
        install_(data.name+"@"+tags[tag], reg, seen, mustHave
                , pkglist, bundles, cb)
      } else {
        log(pkg, "range")
        // prefer the default tag version.
        var defTag = npm.config.get("tag")
          , satis
        defTag = defTag && data["dist-tags"] && data["dist-tags"][defTag]
        if (semver.satisfies(defTag, range)) satis = defTag
        else satis = semver.maxSatisfying(Object.keys(data.versions), range)
  
        if (!satis) return cb(new Error(
          "No satisfying version found for '"+data.name+"'@'"+range+"'"
          +"\nValid install targets for "+data.name+": "
          +installTargets(data)))
        install_(data.name+"@"+satis, reg, seen, mustHave
                , pkglist, bundles, cb)
      }
    })
  }