diff --git a/samples/Prolog/mavis.pl b/samples/Prolog/mavis.pl
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+:- module(mavis, [the/2]).
+/** <module> Optional type declarations
+
+The =mavis= module (because she helps with typing ;-) allows one to
+give optional type declarations to Prolog code. During *development*,
+these declarations throw informative exceptions if values don't match
+types. In *production*, the declarations do nothing.
+
+---+ Why?
+
+We love dynamic types. That's one reason we love Prolog. But sometimes
+types are a helpful tool.  They can:
+
+  * offer documentation to those reading our code
+  * help find errors during development
+  * structure our thinking during development
+  * provide data for static analysis tools
+
+---+ How?
+
+Mavis types are defined using error:has_type/2. We might define an
+=even_integer= type with
+
+==
+error:has_type(even_integer, X) :-
+    0 is X mod 2.
+==
+
+Our code can use that definiton like
+
+==
+%% frobnify(+A, -B)
+frobnify(A, B) :-
+    the(integer, A),
+    the(even_integer, B),
+    B is 2*A.
+==
+
+We can declare types for bound variables, like =A=, and
+not-yet-bound variables, like =B=. The type constraints are implemented
+with freeze/2 so they apply as soon as a variable is bound.
+
+To disable type checking in production, start Prolog with the
+=|-O|= command line argument. A macro eliminates calls to the/2 so they
+have no runtime overhead.
+
+@author Michael Hendricks <michael@ndrix.org>
+@license BSD
+*/
+
+%%  the(+Type, ?Value) is det.
+%
+%	Declare that Value has the given Type.
+%	Succeeds if Value is bound to a value that's compatible
+%	with Type.  Throws an informative exception if Value
+%	is bound to a value that's not compatible with Type.
+%	If Value is not bound, the type check is delayed until
+%	Value becomes bound.
+%
+%	When optimizations are enabled
+%	(=|current_prolog_flag(optimise, true)|= a macro removes =the=
+%	entirely so that it always succeeds.
+:- if(current_prolog_flag(optimise,true)).
+the(_,_).  % avoid "Exported procedure mavis:the/2 is not defined"
+user:goal_expansion(the(_,_), true).
+:- else.
+the(Type, Value) :-
+    freeze(Value, must_be(Type, Value)).
+:- endif.