code: func PaddedBigBytes(bigint *big.Int, n int) []byte {
  	if bigint.BitLen()/8 >= n {
  		return bigint.Bytes()
  	}
  	ret := make([]byte, n)
  	ReadBits(bigint, ret)
  	return ret
  }
docs: // PaddedBigBytes encodes a big integer as a big-endian byte slice. The length
  // of the slice is at least n bytes.