static <C extends Collection<S>> C assertContainsIC(C c, S y) {
  ret assertContainsIC(null, c, y);
}

static <C extends Collection<S>> C assertContainsIC(S msg, C c, S y) {
  if (!cic(c, y))
    fail((msg != null ? msg + ": " : "") + y + " not contained in " + c);
  ret c;
}

ifclass Symbol
static <C extends Collection<Symbol>> C assertContainsIC(C c, Symbol y) {
  ret assertContainsIC(null, c, y);
}

static <C extends Collection<Symbol>> C assertContainsIC(S msg, C c, Symbol y) {
  if (!cic(c, y))
    fail((msg != null ? msg + ": " : "") + y + " not contained in " + c);
  ret c;
}
endif


sS assertContainsIC(S x, S y) {
  if (!cic(x, y))
    fail(quote(y) + " not contained in " + quote(x));
  ret x;
}

Began life as a copy of #1012660