"dict"ionary typing?

dict and tag are my prefered words.

I guess there are two main uses for a dict:
a) as a struct where there are fixed elements of varying types.
b) as a map where there are variable elements of fixed types.

On another note, I've been thinking the functions should restrict the type of dictionary they operate on.

M.multiply(F) := point{x:X, y:Y} :-
X is M.x*F,
Y is M.y*F.

M should be a point.  But how can I make this explicit?  In fact my preference would be this sort of syntax:

multiply(point{x:X0, y:Y0},F, point{x:X, y:Y}):-
X is X0*F,
Y is Y0*F.

?- Point = point{x:3, y:4}, P2 is Point.multiply(3).

or, 
:- type_decl multiply(point{}, number, point{}).
multiply(In,F,Out) :-
Out.x is In.x *F,
Out.y is In.y *F.

and then also have type declaration for dict.  

Alan Baljeu