This would be a breaking change (and thus should be considered before releasing 2.0). Currently there is SingleOrFallback method that takes a delegate. However, I think for consistency with the LINQ API this method should be dropped, and instead provide a FallbackIfEmpty method akin to DefaultIfEmpty, and then use Single, First, Last as needed.

I volunteer to provide the method with tests if you like this approach.

Related to #12 and #67