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

        if (repeat === true || iface === undefined) {
          return harden({ [QCLASS]: 'slot', index });
        } else {
          return harden({ [QCLASS]: 'slot', iface, index });
        }
      }
@param {PassableCap} passable
@param {InterfaceSpec} [iface]
@returns {Encoding}