static AutoCloseable tempSetField_conservativeUndo(O o, S field, O value) {
  fO oldValue = get(o, field);
  set(o, field, value);
  ret () -> { if (eq(get(o, field), value)) set(o, field, oldValue); };
}

static <A> AutoCloseable tempSetField_conservativeUndo(ISetAndGet<A> gs, A value) {
  if (gs == null) null; // convenient null propagation as always in JavaX!
  A oldValue = gs!;
  gs.set(value);
  ret () -> { if (eq(gs!, value)) gs.set(oldValue); };
}

static <A> AutoCloseable tempSetField_conservativeUndo(A value, ISetAndGet<A> gs) {
  ret tempSetField(gs, value);
}

Began life as a copy of #1016243