the formatting to strings/binaries is deferred until the end
would be interesting measure if this change acutally improves performance or will make the sparse folds slower
this performs much better
we stopped a bit too early when only checking for new number permutations (new bitmaps)
add all examples from http://stackoverflow.com/questions/4586814/code-golf-countdown-number-game as testcases
now also taking combinations of subexpressions into account also probably much faster than the digraph based approach.