change-making problem in D (naive implementation)

change.d

#!/usr/bin/rdmd
import std.algorithm : cartesianProduct;
import std.array;
import std.container.rbtree : redBlackTree;
import std.stdio;


int minimum_coins(int target, in int[] denominations)
{
    auto origSet = redBlackTree(target);
    int[] emptyArr = [];
    int k = 0;
    while (!origSet.empty)
    {
        ++k;
        auto newSet = redBlackTree(emptyArr);
        foreach (tup; cartesianProduct(origSet.array, denominations))
        {
            if (tup[0] == tup[1])
            {
                return k;
            }
            else if (tup[0] > tup[1])
            {
                newSet.insert(tup[0] - tup[1]);
            }
            else
                continue;
        }
        origSet = newSet;
    }
    return -1;
}

void main()
{
    auto x = minimum_coins(97, [11, 10, 1]);
    writeln(x);
}

unittest
{
    assert(minimum_coins(97, [11, 10, 1]) == 9);
    assert(minimum_coins(13, [9, 5, 3, 1]) == 3);
    writeln("passed!");
}