1. NEED TO GO THROUGH A FILTER.
S2[
    ISBN[
        CITKEY(@citkey=x1){functional},
        ISBNNUM(@isbn [KEY]=x2){functional}]]
:-
Document(Y1),Book(Y2),[cites:cites](Y2,Y1),
Doc.key(Y1,x1),Book.ISBN(Y2,x2).

2. NEED TO GO THROUGH A FILTER.
S2[
    ISBN[
        CITKEY(@citkey=x1){functional},
        ISBNNUM(@isbn [KEY]=x2){functional}]]
:-
Document(Y1),Book(Y2),isa(Y2,Y1),
Doc.key(Y1,x1),Book.ISBN(Y2,x2).