2803 - new top-level
2804
2805 - 'dump' layer prints to screen

Seems important to be able to use trace for debugging.
But don't try to have the codebase anticipate all possible debuggings.
Nothing wrong in adding a print or cerr once in a while.
2806 - subsetting multiple layers at once

I'm deliberately not writing tests for 'split'. Minimize interfaces with
tests; the more there are, the fewer the possibilities for redrawing
function boundaries.

But we can't just replace readAll with run in all tests; when testing
earlier phases it's nice to be able to provide syms without bindings.

Besides, read vs eval is a hallowed interface boundary. Unlikely we'll
think of even a plausible-seeming twist on it.
2807
2808 - no reason to check the entire trace
2809
2810 - check sublayers when ending in /