/**
 * Joins classes together and ensures atomic declarations of a single group exist.
 * Atomic declarations take the form of `_{group}{value}` (always prefixed with an underscore),
 * where both `group` and `value` are hashes **four characters long**.
 * Class names can be of any length,
 * this function can take both atomic declarations and class names.
 *
 * Input:
 *
 * ```
 * ax(['_aaaabbbb', '_aaaacccc'])
 * ```
 *
 * Output:
 *
 * ```
 * '_aaaacccc'
 * ```
 *
 * @param classes
 */
export default function ax(classNames: (string | undefined | false)[]): string | undefined;