+
+-spec to_undefined(t(A)) ->
+ undefined | A.
+to_undefined(none) -> undefined;
+to_undefined({some, X}) -> X.
+
+-spec validate(t(A), fun((A) -> boolean())) ->
+ t(A).
+validate(none, _) ->
+ none;
+validate({some, X}=T, F) ->
+ case F(X)
+ of false -> none
+ ; true -> T
+ end.