static <A> int lCommonPrefix_lists_ext(L<A> a, L<A> b, int iB) {
  int i = 0, n = min(l(a), l(b)-iB);
  while (i < n && eq(a.get(i), b.get(iB+i)))
    ++i;
  ret i;
}

static <A> int lCommonPrefix_lists_ext(L<A> a, int iA, L<A> b, int iB) {
  int i = 0, n = min(l(a)-iA, l(b)-iB);
  while (i < n && eq(a.get(iA+i), b.get(iB+i)))
    ++i;
  ret i;
}

Began life as a copy of #1029262