code: func FirstBitSet(v *big.Int) int {
  	for i := 0; i < v.BitLen(); i++ {
  		if v.Bit(i) > 0 {
  			return i
  		}
  	}
  	return v.BitLen()
  }
docs: // FirstBitSet returns the index of the first 1 bit in v, counting from LSB.