user8904: Is there a GUI that allows null-moves as legal moves? Of course for engines the null-move heuristic already is an established technique.

user4418: Fritz allows null move entry via CTL+ALT+0. There may also be a menu option to do this.
Scid vs PC also allows this by capturing one king with the other or by using the keyboard to enter "--" i.e. hit the minus key twice.
Hiarcs also uses king takes king to enter a null move. There is also a menu option "Insert/Null Move" in the "Edit" menu, plus a keyboard shortcut of CTL+0

8904: <p>Is there a GUI that allows <em>null-moves</em> as legal moves? Of course for engines the <a href="https://en.wikipedia.org/wiki/Null-move_heuristic" rel="noreferrer">null-move heuristic</a> already is an established technique.</p>
 4418: <p>Fritz allows null move entry via CTL+ALT+0. There may also be a menu option to do this.</p>

<p>Scid vs PC also allows this by capturing one king with the other or by using the keyboard to enter "--" i.e. hit the minus key twice.</p>

<p>Hiarcs also uses king takes king to enter a null move. There is also a menu option "Insert/Null Move" in the "Edit" menu, plus a keyboard shortcut of CTL+0</p>
