Outplace is new in devel, but I wrote "shuffle does the trick". I only used outplace in the example to generate the mutable test array from the range.
And note that NiGui depends on GTK on Linux, so your earlier statement "pure Nim, no depencies" is only valid for windows. There are some GUI libs which do not need SDL or OpenGL. GTK3 needs GTK3 of course. And wNim runs on windows only. First I considered making a GTK3 version of your game, may take a few hours over Christmas. But the game idea is too boring, and I have to do really many other stuff, sorry. Are you considering writing a Nim book? Saw your Python and Go books.
