function goToPage(num) {
                if (paginationService.isRegistered(paginationId) && isValidPageNumber(num)) {
                    var oldPageNumber = scope.pagination.current;

                    scope.pages = generatePagesArray(num, paginationService.getCollectionLength(paginationId), paginationService.getItemsPerPage(paginationId), paginationRange);
                    scope.pagination.current = num;
                    updateRangeValues();

                    // if a callback has been set, then call it with the page number as the first argument
                    // and the previous page number as a second argument
                    if (scope.onPageChange) {
                        scope.onPageChange({
                            newPageNumber : num,
                            oldPageNumber : oldPageNumber
                        });
                    }
                }
            }
Custom "track by" function which allows for duplicate "..." entries on long lists,
yet fixes the problem of wrongly-highlighted links which happens when using
"track by $index" - see https://github.com/michaelbromley/angularUtils/issues/153
@param id
@param index
@returns {string}