/**
 * Donchian Channels.
 * Returns `[upper, lower, middle]` where `upper` is the highest high
 * and `lower` the lowest low over the lookback `period`. `middle` is the
 * midpoint. NaNs propagate when inputs are invalid.
 * @param high High price series
 * @param low Low price series
 * @param period Lookback period (must be > 0)
 * @returns Tuple `[upper, lower, middle]` as Float64Array
 */
export declare function donchian(high: ArrayLike<number>, low: ArrayLike<number>, period: number): [Float64Array, Float64Array, Float64Array];