docs: async function signin (username, password, options = {}) {
      let { agent } = options
      let user = await User.only({ username })
      let sign = user && await UserSign.only({ user })
      let valid = sign && await sign.testPassword(password)
      if (!valid) {
        throw new Error('Signin failed!')
      }
      await user.sync()
      session.signed = user
      return user
    }