/**
 * Create a single string containing all the classnames provided, separated by a space (`" "`).
 * The result will only contain the _last_ atomic style classname for each atomic `group`.
 *
 * ```ts
 * ax(['_aaaabbbb', '_aaaacccc']);
 * // output
 * '_aaaacccc'
 * ```
 *
 * Format of Atomic style classnames: `_{group}{value}` (`_\w{4}\w{4}`)
 *
 * `ax` will preserve any non atomic style classnames (eg `"border-red"`)
 *
 * ```ts
 * ax(['_aaaabbbb', '_aaaacccc', 'border-red']);
 * // output
 * '_aaaacccc border-red'
 * ```
 */
export default function ax(classNames: (string | undefined | null | false)[]): string | undefined;