Kiwi

Suppose we want there to be some high shelves in our game, which the player can't get at unless he's standing on a prop of some kind. (This is a pretty hoary and over-used puzzle, but there may still be occasions when it becomes useful again.)

In order to resolve this, we want to set up a raised supporter kind. When something is on a raised supporter, it should be mentioned to the player only if the player is in the right position (i.e., standing on something) and otherwise omitted from the description entirely.

Note that here we don't continue the activity because we want to completely replace the normal behavior of describing what is on supporters.

Now we also need to prevent the player from interacting with things that are out of reach:

…or restoring things to the shelves while the player is in the wrong position…

And raised supporters shouldn't be searchable from the ground either:

Finally, we need to tackle the case where the player types GET ALL FROM SHELF, because we don't want to list the objects up there if the player can't even see them. We use a rule for deciding whether all includes in order to tell Inform not to consider items that can't be reached, and then we adjust the parser error so that it's a little more instructive than "There are none at all available!", which is what the response would otherwise be: