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 // A set iterator can cope with removing the current entry.

 function test(letters, toRemove) {

     var set = Set(letters);

     toRemove = Set(toRemove);

     var leftovers = [x for (x of set) if (!toRemove.has(x))].join("");

     var log = "";

     for (let x of set) {

         log += x;

         if (toRemove.has(x))

             set.delete(x);

     }

     assertEq(log, letters);

     var remaining = [x for (x of set)].join("");

     assertEq(remaining, leftovers);

 }

 test('a', 'a');    // removing the only entry

 test('abc', 'a');  // removing the first entry

 test('abc', 'b');  // removing a middle entry

 test('abc', 'c');  // removing the last entry

 test('abc', 'abc') // removing all entries