// Use like this: failWithVars(+x, +y);
// Or: failWithVars("bla", +x);
static RuntimeException failWithVars(O... _) {
  S msg = "";
  if (odd(l(_))) {
    msg = str(first(_));
    _ = dropFirst(_);
  }
  ret fail(joinNemptiesWithColonSpace(msg, renderVars_str(_)));
}