The continuation monad. The algorithm is from Wes Dyer http://blogs.msdn.com/b/wesdyer/archive/2008/01/11/the-marvels-of-monads.aspx. The builder approach is from Matthew Podwysocki's excellent Creating Extended Builders series http://codebetter.com/blogs/matthew.podwysocki/archive/2010/01/18/much-ado-about-monads-creating-extended-builders.aspx. Current implementation from Matt's gist at https://gist.github.com/628956