code: func isSource(filePath string) bool {
  	var extension = filepath.Ext(filePath)
  	for _, v := range keep {
  		if v == extension {
  			return true
  		}
  	}
  	return false
  }
docs: // -------------------------------------------------------------------------
  // isSource(path)
  // -------------------------------------------------------------------------
  // predicate function for determining whether a given path should be
  // considered valid source code, based entirely on the extension.  It is
  // assumed that other checks for file type have already been performed.
  // -------------------------------------------------------------------------