/// <summary>
/// Remove a cache entry from the <see cref="IMemoryCache"/> with the given key.
/// </summary>
/// <typeparam name="TSource">The type of the value.</typeparam>
/// <param name="observable">The parent observable.</param>
/// <param name="key">The cache key.</param>
/// <returns>An instance of <see cref="IProviderObservable{TSource}"/>.</returns>
public static IProviderObservable <TSource> RemoveMemoryCacheEntry <TSource>(
this IProviderObservable <TSource> observable, object key) =>
observable.RemoveMemoryCacheEntry(s => key);
