/**
 * Returns a number that represents how many elements in the specified sequence satisfy a condition if present,
 * else the number of items in the collection.
 * @param {Iterable<T>} source A sequence that contains elements to be tested and counted.
 * @param {function(value: T): boolean} [predicate] A function to test each element for a condition.
 */
export declare function count<T>(source: Iterable<T>, predicate?: (value: T) => boolean): number;