enum class Option[A] {
  Some(A)
  None
}

# Only variables of class Option can be assigned to None.

# Type: Option[integer]
var v = Some(10)

# Valid: v is an Option, and any Option can be assigned to None
v = None

# Invalid! v is an Option[integer], not just a plain integer.
v = 10

# Type: integer
var w = 10

# Invalid! Likewise, w is an integer, not an Option.
w = None