+
+-spec of_undefined(undefined | A) ->
+ t(A).
+of_undefined(undefined) -> none;
+of_undefined(X) -> {some, 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.