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

 function test(pairs) {

     print(uneval(pairs));

     var map = Map(pairs);

     var all_keys = '';

     var false_keys = '';

     for (let [k, v] of map) {

         all_keys += k;

         if (!v)

             false_keys += k;

     }

     var log = '';

     for (let [k, remove] of map) {

         log += k;

         if (remove)

             map.delete(k);

     }

     assertEq(log, all_keys);

     var remaining_keys = [k for ([k] of map)].join('');

     assertEq(remaining_keys, false_keys);

 }

 // removing the only entry

 test([['a', true]]);

 // removing the first entry

 test([['a', true], ['b', false], ['c', false]]);

 // removing a middle entry

 test([['a', false], ['b', true], ['c', false]]);

 // removing the last entry

 test([['a', false], ['b', false], ['c', true]]);

 // removing all entries

 test([['a', true], ['b', true], ['c', true]]);