The parser asks if you mean the back door or the white door, even when the white door is gone

>LOOK
Herb Garden

A tall fence protects the neat rows of herbs from predators. The only exit is
the open back door of the cottage, leading west.

Another giant toadstool has taken root in a patch of thyme.

A triangular shadow lies across the ground. Its sharp point rests exactly on the
toadstool.

A pile of refuse lies in the corner.

>EXAMINE WHITE DOOR
The white door you saw here before is gone now.

>EXAMINE DOOR
Which door do you mean, the back door or the white door?

The game does have a GENERIC-DOOR-F routine to help disambiguate between doors, but it's used mostly to make sure that you're referring to only one door at a time, and to make "OPEN" default to the closed door, etc. Perhaps it should do something like this? It seems to work when I try it:

diff --git a/misc.zil b/misc.zil
index cdfb718..71d5607 100644
--- a/misc.zil
+++ b/misc.zil
@@ -1377,6 +1377,9 @@
         <RTRUE>>

 <ROUTINE GENERIC-DOOR-F (TABLE "AUX" (PTR 1) (DOOR <>) LEN X)
+        <COND (<AND <HERE? ,IN-GARDEN>
+                    <IS? ,TS4-DOOR ,NOALL>>
+               <RETURN ,GARDEN-DOOR>)>
         <COND (<OR <EQUAL? ,P-NAM ,W?DOORS ,W?DOORWAYS>
                    <EQUAL? ,P-XNAM ,W?DOORS ,W?DOORWAYS>>
                <MORE-THAN-ONE "door">

I put it first in the routine so that "EXAMINE DOORS" wouldn't incorrectly claim that there are more than one.

the-infocom-files / trinity

The parser asks if you mean the back door or the white door, even when the white door is gone #88