function $ref (received, expected) {
  const pass = (
    isRef(received)
    && (expected === void 0 || this.equals(received.value, expected))
  )

  return {
    pass,
    message: () =>
      `expected ${ this.utils.printReceived(
        received.value
      ) } to${ this.isNot ? ' not' : '' } be a ref/computed${ expected !== void 0
        ? ` and be equal to ${ this.utils.printExpected(expected) }`
        : ''
      }`
  }
}
Examples:
  expect(variable).$ref()
  expect.$ref()
  expect.$ref(<someValue>)
  expect.$ref(expect.any(Number))