encodeSlotToSmallcaps = (prefix, passable, iface = undefined) => {
        const { index, repeat } = encodeSlotCommon(passable);

        // TODO explore removing this special case
        if (repeat === true || iface === undefined) {
          return `${prefix}${index}`;
        }
        return `${prefix}${index}.${iface}`;
      }
@param {string} prefix
@param {PassableCap} passable
@param {InterfaceSpec} [iface]
@returns {string}