code: func CheckProofOfWork(block *btcutil.Block, powLimit *big.Int) error {
  	return checkProofOfWork(&block.MsgBlock().Header, powLimit, BFNone)
  }
docs: // CheckProofOfWork ensures the block header bits which indicate the target
  // difficulty is in min/max range and that the block hash is less than the
  // target difficulty as claimed.