func (db *DB) removeDepLocked(x finalCloser, dep any) func() error {
	xdep, ok := db.dep[x]
	if !ok {
		panic(fmt.Sprintf("unpaired removeDep: no deps for %T", x))
	}

	l0 := len(xdep)
	delete(xdep, dep)

	switch len(xdep) {
	case l0:
		// Nothing removed. Shouldn't happen.
		panic(fmt.Sprintf("unpaired removeDep: no %T dep on %T", dep, x))
	case 0:
		// No more dependencies.
		delete(db.dep, x)