function krest(r)
  for s = 'owns'
    marking(r, s);
    endfor
  r.mark();
endfunction

function marking(r, s)
l = 0;
  while !r.is_bord(s)
    l += 1;
    r.step(s);
    r.mark();
    endwhile
  for k = 1:l
    r.step(reverse(s));
    endfor
endfunction

function s = reverse(s)
  if s == 'o'
    s = 'w';
  elseif s == 'w'
    s = 'o';
  elseif s == 'n'
    s = 's';
  elseif s == 's'
    s = 'n';
    endif
endfunction