Section 3(c) - The side door
The side door is an openable locked door. It is east of Du Monde's and west of Side Corridor. The description of the side door is "A plain, nondescript door leading [if the player is in Du Monde's]east[otherwise]west[end if]." The matching key of the side door is the heavy key. Instead of opening or unlocking or entering the side door while the player is in Du Monde's and Du Monde is alive, say "'That leads to my storerooms, or to the flat - neither of which I believe should be of any interest to you...'"
The description of the heavy key is "A heavy key with 1428B engraved along its length. It must have fallen from Du Monde's pocket."
Instead of attacking the side door, say "The door is too solid to knock down."