op !=

pub(api) interface Storage {
  fn get(k: str) -> bytes?  io
  var fn put(k: str, v: bytes)  io
  fn has(k: str) -> bool  io => get(k) != none     // a default method
}

interface ReadWrite { embed Reader; embed Writer }
interface Container<T> { type Item = T; fn items() -> iter<T> }

@explicit interface Persistent { fn key() -> str }

assert FileStorage: Storage                      // static check

pub(api) data User {
  pub(api) id: UserId
  pub(api) name: Name
  pub(api) email: Email?           @since(2)
  role: Role = Role.User
  createdAt: Time = now()
  invariant name != "admin" || role == Role.Admin
}