How do you purge a removed app? or delete snapshot after app removal

As I was writing the question, I quickly realized I could use the snapshot commands:

$ snap help

...
Snapshots: saved, save, check-snapshot, restore, forget
...

Looks like I need to find the saved snapshots with saved and then forget it.

$ sudo snap saved

Set  Snap      Age    Version  Rev  Size    Notes
1    app-name  4m30s  1.2.0    25   1.89MB  auto

$ sudo snap forget 1

Works.

Hope this obvious answer could help others.

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