static int indexOfIntPairInContentsIndexedList(IContentsIndexedList2<Int> l, Int pa, Int pb, int idx) {
  SortedSet<HasIndex> setA = l.indicesOf_treeSetOfHasIndex(pa);
  if (setA == null) ret -1;
  SortedSet<HasIndex> setB = l.indicesOf_treeSetOfHasIndex(pb);
  if (setB == null) ret -1;
  
  // Take size now because size of a tailSet is costly to compute
  if (setA.size() <= setB.size()) {
    // quickly skip entries left of idx
    if (idx > 0)
      setA = setA.tailSet(HasIndex(idx));

    // scan set, check if it's a pair
    int n = l.size();
    for (HasIndex hi : setA) {
      int i = hi.idx;
      if (i+1 < n && eq(l.get(i+1), pb))
        ret i;
    }
  } else {
    // quickly skip entries left of idx+1
    setB = setB.tailSet(HasIndex(idx+1));
    
    // scan set, check if it's a pair
    for (HasIndex hi : setB) {
      int i = hi.idx;
      if (eq(l.get(i-1), pa))
        ret i-1;
    }
  }
  
  ret -1;
}