But not that doing so will cause the program to have an exponential runtime as each new ys must be repeatedly traversed to append a [y].. The alternative is to *unfold* your list by recursing on the right hand side of the (:) to add new elements.

On Thu, Jan 21, 2016 at 5:43 AM Doug McIlroy <doug@cs.dartmouth.edu> wrote:

Each time you find another good 9-mer, you add it to
the head of the list. This means that the ultimate
list will be in reverse order of discovery: the first element
to be printed is the last one to be found.  To get
output in the order it was discovered, build the
output by ys++[y] rather than y:ys.
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