code: func Open(dirpath string, snap walpb.Snapshot) (*WAL, error) {
  	return openAtIndex(dirpath, snap, true)
  }
docs: // Open opens the WAL at the given snap.
  // The snap SHOULD have been previously saved to the WAL, or the following
  // ReadAll will fail.
  // The returned WAL is ready to read and the first record will be the one after
  // the given snap. The WAL cannot be appended to before reading out all of its
  // previous records.