A woolen mat

this is maybe a silly question, but what’s wrong with this:

Blue room is a room.

A dog is animal. The description is "a blue dog". a dog is in the blue room.
Red room is a room.
Blue door is a lockable door. It is locked.
The matching key is metal key.
A blue door is west of blue room and east of red room.
A metal key is under a mat.
A mat is a thing. The description of the mat is "A woolen mat." The mat is in the blue room.

The sentence ‘A metal key is under a mat’ appears to say two things are the same - I am reading ‘metal key’ and ‘under a mat’ as two different things, and therefore it makes no sense to say that one is the other

Inform doesn’t natively understand what ‘under’ is. You have to program an under relation: this is explained in Chapter 6.6 of the Recipe Book 6.6. Looking Under and Hiding

Thanks a lot. I’ll check that.