Problem with unifying values

Help!
1

I’m using: SWI-Prolog version 7.6.4

I have a list containing variables and atoms. The variables can also be inside a “not”. The predicate assign should bind true/false to them.

Query:
assign([true,false,X,not(Y)]).

One Solution I get:

X = not(true),
Y = true

I don’t want to get not(true) or not(false) as possible solutions. Instead I want to have only true/false as possible assignments.

My code looks like this:

assign([]).
assign([not(H)|T]) :-
    var(H),
    bind(H),
    assign(T).
assign([H|T]) :-
    var(H),
    bind(H),
    assign(T).
assign([H|T]) :-
    ground(H),
    assign(T).

bind(true).
bind(false).