_verifyManagedOBChecksum (symbol, prec, cs) {
    const ob = this._losslessOrderBooks[symbol]

    if (!ob) return null

    const localCS = ob instanceof OrderBook
      ? ob.checksum()
      : OrderBook.checksumArr(ob, prec === 'R0')

    return localCS !== cs
      ? new Error(`OB checksum mismatch: got ${localCS}, want ${cs}`)
      : null
  }
@param {string} symbol - symbol for order book
@param {string} prec - precision
@param {number} cs - expected checksum
@returns {Error} err - null if none
@private