/**
 * Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.
 * @typeparam TSource The type of the elements of source.
 * @param src An Iterable to return a single element from.
 * @param predicate A function to test an element for a condition.
 * @returns The single element of the input sequence that satisfies a condition.
 */
export declare function single<TSource>(src: Iterable<TSource>, predicate?: (item: TSource, index: number) => boolean): TSource;
/**
 * Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.
 * @typeparam TSource The type of the elements of source.
 * @param src An Iterable to return a single element from.
 * @param predicate A function to test an element for a condition.
 * @returns The single element of the input sequence that satisfies a condition.
 */
export declare function singleOrDefault<TSource>(src: Iterable<TSource>, predicate?: (item: TSource, index: number) => boolean): TSource | null;
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