CRT Combat Model

[Jim Kingdon]

> the first 3-3 attacks at 3-2 odds, the 1 hp 1-3 goes to 0 hp and
> becomes a wreck with defend value 1 (continuing to occupy the city and
> forcing a second attack), the second 3-3 overruns at 3-2 odds, making
> the wreck vanish

Have you looked at wrecked-type?  For an example, see monster.g,
especially:

(add building wrecked-type BB)

(add BB wrecked-type WB)

(add WB wrecked-type R)