if (*cp++ == ':') /* colon in a non-colon-sep list item, needs doubling */
{
yield = string_catn(yield, &size, &ptr, US"::", 2);
if (*cp++ == ':') /* colon in a non-colon-sep list item, needs doubling */
{
yield = string_catn(yield, &size, &ptr, US"::", 2);