code: func (t *tOps) offsetOf(f *tFile, key []byte) (offset int64, err error) {
  	ch, err := t.open(f)
  	if err != nil {
  		return
  	}
  	defer ch.Release()
  	return ch.Value().(*table.Reader).OffsetOf(key)
  }
docs: // Returns approximate offset of the given key.