How to use the levelup.call function in levelup

options.db = txdown(db._levelup, options.createLock)
    location = db.location
  } else if (isFunction(db.sublevel) && isFunction(db.levelup)) {
    // db is sublevelup, get its levelup
    this._levelup = db.levelup()
    options.db = txdown(db.levelup(), options.createLock)
    location = db.location
  } else {
    // db is LevelUP, wrap txdown
    this._levelup = db
    options.db = txdown(db)
    location = ''
  }

  // LevelUP.call(this, options.db(location), options)
  LevelUP.call(this, location, options)

  var self = this
  this.once('closed', function () {
    self.emit('end', self.db._error)
    self.emit('release', self.db._error)
  })
}

function Db (location) {
  // Initialize necessary methods/properties from levelup in this instance
  levelup.call(this, location)
}