/**
 * Bollinger Bands (BB).
 * Returns `[middle, upper, lower]` where `middle` is the SMA, and `upper`/`lower`
 * are `middle ± mult * stddev`. Uses a NaN-aware or dense `rollstdev` depending
 * on input; positions before the window fills are NaN.
 * @param source Input series
 * @param period Window length (must be > 0)
 * @param mult Standard-deviation multiplier
 * @returns Tuple `[middle, upper, lower]` as Float64Array
 */
export declare function bb(source: ArrayLike<number>, period: number, mult: number): [Float64Array, Float64Array, Float64Array];