code: func ParseBig256(s string) (*big.Int, bool) {
  	if s == "" {
  		return new(big.Int), true
  	}
  	var bigint *big.Int
  	var ok bool
  	if len(s) >= 2 && (s[:2] == "0x" || s[:2] == "0X") {
  		bigint, ok = new(big.Int).SetString(s[2:], 16)
  	} else {
  		bigint, ok = new(big.Int).SetString(s, 10)
  	}
  	if ok && bigint.BitLen() > 256 {
  		bigint, ok = nil, false
  	}
  	return bigint, ok
  }
docs: // ParseBig256 parses s as a 256 bit integer in decimal or hexadecimal syntax.
  // Leading zeros are accepted. The empty string parses as zero.