Fixed

Thanks!

You are right, there is a problem in randomtests: "they code only letters a-z, A-Z" so the string must not begin with a character outside a-z, A-Z (here, if it begins with "|" this character should not be coded and the prefix would be "||" hence the shift would not be known). I modified the randomtests. I hope it will work now for you! Unfortunately it seems that you are the first to try the F# translation:-(. Thanks for your post.