code: func MustParseBig256(s string) *big.Int {
  	v, ok := ParseBig256(s)
  	if !ok {
  		panic("invalid 256 bit integer: " + s)
  	}
  	return v
  }
docs: // MustParseBig256 parses s as a 256 bit big integer and panics if the string is invalid.