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

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

Began life as a copy of #1030263