[Haskell-cafe] My knight's tour does not seem to end

[Stephen Tetley]

Isn't mark is always increasing the size of the board? - I haven't run
the code but I if this is the behaviour you want 'mark' is probably
not a good name (I'd expect mark to be returning something the same
size but with marked elements).

In _mark_ the before and after are always splits, so the concatenation
on the first line of the definition of _mark_ is taking the splits and
adding more between them thus expanding the board.