A trail whose screen on top is Top. The ~screen.top marker is type-only
(never present at run time): in the process that wrote it, retrace narrows
its result to that screen; a trail read back from JSON is unknown and
retraces to the screen set's whole union.
A trail whose screen on top is
Top. The~screen.topmarker is type-only (never present at run time): in the process that wrote it,retracenarrows its result to that screen; a trail read back from JSON isunknownand retraces to the screen set's whole union.