function helperForTestIsSubsetOf(a, b, c, d) {
  const s = new StructsSet([a, b, c]);

  assertTrue('set <= itself', s.isSubsetOf(s));
  assertTrue('set <= same set', s.isSubsetOf(new StructsSet([a, b, c])));
  assertTrue('set <= its clone', s.isSubsetOf(s.clone()));
  assertTrue('set <= array of same elements', s.isSubsetOf([a, b, c]));
  assertTrue(
      'set <= array of same elements in different order', s.equals([c, b, a]));

  assertTrue(
      'set <= Set([a, b, c, d])', s.isSubsetOf(new StructsSet([a, b, c, d])));
  assertTrue('set <= [a, b, c, d]', s.isSubsetOf([a, b, c, d]));
  assertTrue('set <= [a, b, c, c]', s.isSubsetOf([a, b, c, c]));

  assertFalse('set !<= Set([a, b])', s.isSubsetOf(new StructsSet([a, b])));
  assertFalse('set !<= [a, b]', s.isSubsetOf([a, b]));
  assertFalse('set !<= Set([c, d])', s.isSubsetOf(new StructsSet([c, d])));
  assertFalse('set !<= [c, d]', s.isSubsetOf([c, d]));
  assertFalse(
      'set !<= Set([a, c, d])', s.isSubsetOf(new StructsSet([a, c, d])));
  assertFalse('set !<= [a, c, d]', s.isSubsetOf([a, c, d]));
  assertFalse('set !<= [a, a, b]', s.isSubsetOf([a, a, b]));
  assertFalse('set !<= [a, a, b, b]', s.isSubsetOf([a, a, b, b]));
}
Helper method for testIsSubsetOf().
@param {Object} a First element to use in the tests.
@param {Object} b Second element to use in the tests.
@param {Object} c Third element to use in the tests.
@param {Object} d Fourth element to use in the tests.