code: func StripVirtualEnvPaths(env environ.Env) (ret environ.Env, pruned []string) {
  	ret = env.Clone()
  	path := filepath.SplitList(env.GetEmpty("PATH"))
  	newPath := make([]string, 0, len(path))
  	for _, entry := range path {
  		if _, err := os.Stat(filepath.Join(entry, "activate_this.py")); err == nil {
  			pruned = append(pruned, entry)
  		} else {
  			newPath = append(newPath, entry)
  		}
  	}
  	ret.Set("PATH", strings.Join(newPath, string(filepath.ListSeparator)))
  	return
  }
docs: // StripVirtualEnvPaths looks for all $PATH elements which are the `BinDir` of
  // a VirtualEnv deployment (created by VPython or not), and removes them. These
  // directories contain a `python` interpreter and various scripts (like
  // activate).
  //
  // This uses VirtualEnv's "<BinDir>/activate_this.py" file to identify
  // VirtualEnvs, which is installed by all known versions of VirtualEnv.
  //
  // This returns a modified copy of env and a list of pruned paths (if any).