gczeal(0);
let W = newGlobal({newCompartment: true});
W.eval(`var h2 = {s: Symbol('s')};`);
addMarkObservers([W.h2.s]);
W.eval(`grayRoot().push(h2); h2 = null;`);
gc();
gc();
printErr("GC1 (full): symbol=" + getMarks()[0]);
schedulezone(W);
startgc(1);
while (gcstate() != "Mark" && gcstate() != "NotActive") gcslice(1);
printErr("GC2 (incremental, zone W only, atoms not collected): state=" + gcstate());
W.eval(`var h = grayRoot()[0]; var keep = {}; keep.s = h.s; h.s = 1;`);
while (gcstate() != "NotActive") gcslice(1000);
printErr("GC2 done: symbol=" + getMarks()[0] + " (referenced from black object W.keep)");
W.eval(`String(keep.s);`);