static <A> AutoCloseable tempSetThreadLocalIfNecessary(ThreadLocal<A> tl, A a) {
  if (tl == null) null;
  A prev = tl.get();
  if (eq(prev, a)) null;
  tl.set(a);
  ret autoCloseable { tl.set(prev); };
}

ifclass BetterThreadLocal
static <A> AutoCloseable tempSetThreadLocalIfNecessary(BetterThreadLocal<A> tl, A a) {
  if (tl == null) null;
  A prev = tl.get();
  if (eq(prev, a)) null;
  tl.set(a);
  ret autoCloseable { tl.set(prev); };
}
endif

Began life as a copy of #1012933