    %      +------------------------------+
    %      |   test for Prolog source     |
    %      |______________________________|

    isinteger(X, L) :- type(X, int, L).

    % booleans
    type(true, bool, _).
    type(false, bool, _).

    type( and(X,Y), bool, L) :- isboolean(X,L), isboolean(Y,L).
    type( or(X,Y), bool, L) :- isboolean(X,L), isboolean(Y,L).
    type( not(X), bool, L) :- isboolean(X,L).

    type( succ(X), int, L ) :- isinteger(X,L).
    type( pred(X), int, L ) :- isinteger(X,L).
    type( iszero(X), bool, L ) :- isinteger(X,L).
    type( X < Y, bool, L ) :- isinteger(X,L), isinteger(Y,L).

    % is_member(X,L) check whether X is in the list
    % by using unification with occur check

    is_member(_, []) :- fail.
    is_member(X, [Y | _]) :- unify(X,Y).
    is_member(X, [_ | List]) :- is_member(X, List).