Containers - Contains Moved Thing Check

I haven’t found this in the documentation, perhaps I just need a url.

Here is what I’m attempting to do:
I have a fireplace with some hot embers in it. There is a broken shelf. Hanging inside the fireplace is a pot that the player cannot see.

The ‘puzzle’ idea is: the player takes the broken shelf, puts it in the fireplace (so they aren’t burned by the coals). If the board is there they can look in the fireplace at which point they discover the hidden pot.

What is the syntax used to see if the shelf is moved into the fireplace, so I can use this check for the LookIn action?

Thanks!

if(whiteFeather.isIn(hall))
    "The white feather must be around here somewhere. ";

More examples in Learning TADS 3, section 5.1.4 Testing for Containment.

Excellent! Thank you so much. I hadn’t found that section. That is exactly what I needed.