code: func (m *DefaultScriptSourceManager) InstallOptional(scripts []string, dstDir string) []api.InstallResult {
  	result := []api.InstallResult{}
  	for _, script := range scripts {
  		installed := false
  		failedSources := []string{}
  		for _, e := range m.sources {
  			detected := false
  			h := e.(ScriptHandler)
  			h.SetDestinationDir(dstDir)
  			if r := h.Get(script); r != nil {
  				if err := h.Install(r); err != nil {
  					failedSources = append(failedSources, h.String())
  					// all this means is this source didn't have this particular script
  					glog.V(4).Infof("script %q found by the %s, but failed to install: %v", script, h, err)
  				} else {
  					r.FailedSources = failedSources
  					result = append(result, *r)
  					installed = true
  					detected = true
  					glog.V(4).Infof("Using %q installed from %q", script, r.URL)
  				}
  			}
  			if detected {
  				break
  			}
  		}
  		if !installed {
  			result = append(result, api.InstallResult{
  				FailedSources: failedSources,
  				Script:        script,
  				Error:         fmt.Errorf("script %q not installed", script),
  			})
  		}
  	}
  	return result
  }
docs: // InstallOptional downloads and installs a set of scripts into dstDir, the result is a
  // map of scripts with detailed information about each of the scripts install process