static <A, B> L<A> asForPairsWithB(B b, Iterable<Pair<A, B>> l) {
  new L<A> out;
  fOr (Pair<A, B> p : l)
    if (eq(pairB(p), b))
      out.add(pairA(p));
  ret out;
}

static <A, B> L<A> asForPairsWithB(Iterable<Pair<A, B>> l, B b) {
  ret asForPairsWithB(b, l);
}

Began life as a copy of #1027828