> (b Box[T]) Map[U any](f func(T) U) Box[U]

  Map method 
    of b (of type Box[T]) 
      that takes f 
      (of type function that takes value of type T and returns value of type U (which could be any type))
      and returns value of type Box[U]
  is defined as follows
    return Box[U]{v: f(b.v)}

  func[U any] b:Box[T].Map(f:func(T)->U)->Box[U]:
    return {v: f(b.v)}

  func[U any] Box[T].Map(f:func(T)->U)->Box[U]:
    return {v: f(this.v)}

  // maybe all of the types could be inferred from usage?
  func Box[].Map(f):
    return Box[]{v: f(this.v)}

Eh... I think you'd need to avoid generics altogether.