michael@0: // A set iterator can cope with removing the current entry.
michael@0: 
michael@0: function test(letters, toRemove) {
michael@0:     var set = Set(letters);
michael@0:     toRemove = Set(toRemove);
michael@0: 
michael@0:     var leftovers = [x for (x of set) if (!toRemove.has(x))].join("");
michael@0: 
michael@0:     var log = "";
michael@0:     for (let x of set) {
michael@0:         log += x;
michael@0:         if (toRemove.has(x))
michael@0:             set.delete(x);
michael@0:     }
michael@0:     assertEq(log, letters);
michael@0: 
michael@0:     var remaining = [x for (x of set)].join("");
michael@0:     assertEq(remaining, leftovers);
michael@0: }
michael@0: 
michael@0: test('a', 'a');    // removing the only entry
michael@0: test('abc', 'a');  // removing the first entry
michael@0: test('abc', 'b');  // removing a middle entry
michael@0: test('abc', 'c');  // removing the last entry
michael@0: test('abc', 'abc') // removing all entries
michael@0: