You can check what’s built-in by using the ACTIONS testing command.
>actions
Actions listing on.
>tie rock
(to the table)
[tying the rock to the table]
You would achieve nothing by this.
[tying the rock to the table - failed the block tying rule]
>untie rock
That’s not a verb I recognise.
So Inform doesn’t have a built-in verb for “untie”. You’ll have to define your own. If you want to untie only a single object:
Untying is an action applying to one thing.
Understand "untie [something]" as untying.
Check untying (this is the block untying rule): say "There's nothing to be achieved by that." instead.
If you want to specify two objects, just like with tying:
Untying it from is an action applying to two things.
Understand "untie [something] from [something]" as untying.
Check untying something from something (this is the block untying rule): say "There's nothing to be achieved by that." instead.
Now you can write “instead of untying” rules, just like you can write “instead of tying” currently.