A checkpoint narrowed to the screen Top by ownsCheckpoint. The
~screen.top marker is type-only (never present at run time): checkpoint()
returns the generic ScreenCheckpoint (the screen is known at run time), and
the screen's synchronous guard proves which screen it belongs to.
A checkpoint narrowed to the screen
TopbyownsCheckpoint. The~screen.topmarker is type-only (never present at run time):checkpoint()returns the genericScreenCheckpoint(the screen is known at run time), and the screen's synchronous guard proves which screen it belongs to.