code: function (other) {
          assertNotNull(other);
  
          var c = collectionCount(other);
  
          if (c !== -1) {
              if (this.count() === 0) {
                  return c > 0;
              }
  
              // faster if other is a hashset (and we're using same equality comparer)
              else if (areEqualityComparersEqual(this, other)) {
                  if (this.count() >= c) {
                      return false;
                  }
  
                  // this has strictly less than number of items in other, so the following
                  // check suffices for proper subset.
                  else {
                      return containsAllElements(other, this);
                  }
              }
          }
  
          var result = checkUniqueAndUnfoundElements(this, other, false);
          return (result.uniqueCount === this.count() && result.unfoundCount > 0);
      }
docs: Determines whether the current set is a proper (strict) subset of a specified collection.
  @param {Iterable} other The collection to compare to the current set.
  @returns {Boolean}