/**
 * Flatten an array by one level.
 *
 * @param arr - Array of arrays
 * @returns Flattened array
 *
 * @example
 * flattenFast([[1, 2], [3, 4]]) // [1, 2, 3, 4]
 */
export declare function flattenFast<T>(arr: readonly (T | readonly T[])[]): T[];
/**
 * Recursively flatten an array to any depth.
 *
 * @param arr - Nested array
 * @param depth - Maximum depth to flatten (default: Infinity)
 * @returns Flattened array
 *
 * @example
 * flattenDeepFast([1, [2, [3, [4]]]]) // [1, 2, 3, 4]
 * flattenDeepFast([1, [2, [3, [4]]]], 2) // [1, 2, 3, [4]]
 */
export declare function flattenDeepFast<T>(arr: readonly unknown[], depth?: number): T[];
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