He told met to post the letter, or burn it -- I forget.

The phrase "I forget" usually cancels a free-choice.

"The hotel doesn't allow dogs or cats -- I forget which".

Notably, out of

!p

one can derive

!p v do nothing at all.

"Close the door!"

"Close the door or don't do anything at all!"

Grice is charismatic and charitable. He notes that it's best to understand an imperative as the desire on the part of the utterer that the addressee will form a desire (e.g. to close the door). Surely, since this is a free world, there is always the chance that the addressee WON'T close the door. The satisfaction conditions, which is what the Semantics establish, remain fixed, though.