How to do fluent enumerable method mentioned in todo (@bathroomcommits)

DemandSetup.cs

	public class Demand
	{
		public static DemandResult That(Func<bool> predicate) => new DemandResult(predicate);

		public static void That(bool predicate, string reason)
		{
			if (!predicate) throw new InvalidOperationException(reason);
		}

		public static DemandSetup<IEnumerable<T>> That<T>(IEnumerable<T> source) => new EnumerableDemand<T>(source);
	}

	public interface DemandSetup<T>
	{
		T Source { get; }
	}

	internal sealed class EnumerableDemand<T> : DemandSetup<IEnumerable<T>>
	{
		public IEnumerable<T> Source { get; }

		public EnumerableDemand(IEnumerable<T> source) { Source = source; }
	}

	public static class EnumerableDemands
	{
		public static void NonEmpty<T>(this DemandSetup<IEnumerable<T>> setup, string reason)
		{
			if (!setup.Source.Any()) throw new InvalidOperationException(reason);
		}
	}

Nice! It's a lot closer to what I had in mind; thanks!