remove lazy, fix SRule to String
iterative deepening search and append model
more fun with append
do not treat certain choices as depth increase
lazy lists with random tails. not easy without additional memoization
memoizing continuations

append now yields correct results for lists with
probabilistic tails. for each x.flatMap(f), f is
memoized per exploration path. is that enough or
are there cases where the scheme breaks down?
another append example. update check file
Merge pull request #4 from TiarkRompf/topic-virt-patmat-2.10.0

iterative deepening search and append example