How to implement subtraction using only loop and increment

Question

This is an interview question. We have only two constructs

1. loop(a) means loop for a times.
2. increment(a) increments a.

Thus to implement a+b one could write

loop(a) {inc(b)} 
return b;

The question is how to implement a-b.

Answer 1

How about;

a = 10
b = 8
result = 0

loop(b) {
   last = 0
   times = 0;
   loop(a) {
      last = times
      times = inc(times)
   }
   result = a = last
}

result is 2

Js eg;

var a = 10;
var b = 8;
var result;

for (var _b = 0; _b < b; _b++) {
    var last = 0, times = 0, loopa = 0;
    for (var _a = 0; _a < a; _a++) {
        last = times;
        times = inc(times);
    }
    result = a = last;
}

function inc(i) {
    return i + 1;
}

print(result) // 2