code: func sub2(a, b, carry int32) (int32, int32) {
  	diff := a - b - carry
  	if diff < 0 {
  		carry = 1
  		diff += wordBase
  	} else {
  		carry = 0
  	}
  	if diff < 0 {
  		diff += wordBase
  		carry++
  	}
  	return diff, carry
  }
docs: // sub2 subtracts b and carry from a, returns the diff and new carry.
  // the new carry may be 2.