Option

• opt : type -> type
  The type constructor for optional values.
• none : opt A
  An optional value where a value is not present.
• some : A -> opt A
  An optional value where a value is present.
• get_opt (opt A,A)
  mode get_opt(in,out)
  Succeeds if the first argument is $some(A)$ and the second argument is $A$.