static <A> A popFirst(L<A> l) {
  if (empty(l)) null;
  A a = first(l);
  l.remove(0);
  ret a;
}

static <A> A popFirst(Cl<A> l) {
  if (empty(l)) null;
  A a = first(l);
  l.remove(a);
  ret a;
}

static <A, B> Pair<A, B> popFirst(Map<A, B> map) {
  if (map == null) null;
  var it = map.entrySet().iterator();
  if (!it.hasNext()) null;
  var p = mapEntryToPair(it.next());
  it.remove();
  ret p;
}

static <A> L<A> popFirst(int n, L<A> l) {
  L<A> part = cloneSubList(l, 0, n);
  removeSubList(l, 0, n);
  ret part;
}

static <A> AppendableChain<A> popFirst(AppendableChain<A> a) {
  ret a?.popFirst();
}

Began life as a copy of #1002780