code: func (x *Index) schemaVersion() int {
  	schemaVersionStr, err := x.s.Get(keySchemaVersion.name)
  	if err != nil {
  		if err == sorted.ErrNotFound {
  			return 0
  		}
  		panic(fmt.Sprintf("Could not get index schema version: %v", err))
  	}
  	schemaVersion, err := strconv.Atoi(schemaVersionStr)
  	if err != nil {
  		panic(fmt.Sprintf("Bogus index schema version: %q", schemaVersionStr))
  	}
  	return schemaVersion
  }
docs: // schemaVersion returns the version of schema as it is found
  // in the currently used index. If not found, it returns 0.