static <A> Map<A, Bool> trueFalseMapFromTwoLists(Collection<A> trueList, Collection<A> falseList) {
  new Map<A, Bool> map;
  for (A a : unnull(trueList))
    map.put(a, true);
  for (A a : unnull(falseList))
    map.put(a, false);
  ret map;
}