User (
  name: str,
  age: int,
  email: str,
  bday: date,
  friends: []User,
  posts: []Post,
  comments: []Comment,
)

Post (
  text: str,
  at: datetime,
  from: User,
  comments: []Comment,
)

Comment (
  text: str,
  at: datetime,
  from: User,
  of: Post,
)
