Calling an onion "a single yellow onion" when (and only when) it is being listed as the sole content of a room or container.

"Unpeeled"
Scullery is a room. A sack is carried by the player. The sack contains a yellow onion. The player carries a cork.
Before printing the name of the onion while listing contents:
   if the holder of the onion contains exactly 1 thing, say "single ".
Test me with "i / put cork in sack / i".
Test me with "i / put cork in sack / i".
Scullery

>(Testing.)

>[1] i
You are carrying:
a sack
a single yellow onion
a cork

>[2] put cork in sack
You put the cork into the sack.

>[3] i
You are carrying:
a sack
a cork
a yellow onion