Oh oh ... I just tried something else:
{ if true "1" "2" }
{ if false "1" "2" }
both show 1.

TRUE, FALSE did not work, either (still showed 1, 1).
Neither did 1 for true, 0 for false (unknown opcode).

I guess I must be using booleans in Word's mail merge
entirely wrong :-( .

Are there no boolean literals?
Is it not possible to assign booleans to variables
(bookmarks)?

But { = AND (true, false) } worked, no matter the
capitalization and also with 1 and 0, same for OR. So
true, false must be the correct literals.

This does not make any sense to me.

Monika.