The following old document is from Niels' personal file at DJOE:

old document (click to enlarge)

The document isn't dated but judging from content and printer font the program it refers to must have been written on the original EXIDY machine, which would put it somewhere in the 1979-1983 timeframe.

We have a full transcription of the document; it describes a program for playing BOTER-KAAS EN EIEREN (tic-tac-toe) against the computer, based on a theoretical analysis of the game.

Especially interesting is the attribution at the bottom of the document:

Met dank aan L.Rooijakkers die een oude  routine  opviste om de
kruisjes en rondjes op het scherm te zetten.

The "oude routine" that Niels refers to almost certainly was something like this:

1000 REM Put char C in the game board at position X, Y
1010 Y1 = nn + 2 * Y : X1 = nn + 2 * X
1020 POKE -3968 + 64 * Y1 + X1, C

where the nn were constants positioning the board on the screen (the EXIDY had a 64x30 memory-mapped video display whose first line started at memory address F080 hex, corresponding to -3968 in 16-bit two's complement).

The fact that Niels did the theoretical dissection of the game himself but relied on others for low-level machine interaction is typical for him at that time. It also shows that he had no problem placing credit where credit was due.