code: func Checksum(ctx context.Context, ref cache.ImmutableRef, path string, followLinks bool) (digest.Digest, error) {
  	return getDefaultManager().Checksum(ctx, ref, path, followLinks)
  }
docs: // Layout in the radix tree: Every path is saved by cleaned absolute unix path.
  // Directories have 2 records, one contains digest for directory header, other
  // the recursive digest for directory contents. "/dir/" is the record for
  // header, "/dir" is for contents. For the root node "" (empty string) is the
  // key for root, "/" for the root header