theorem
 for f being Function, x,y being object
 holds card(f+~(x,y)) = card f
proof let f be Function,x,y be object;
 thus card(f+~(x,y)) = card dom(f+~(x,y)) by Th60
    .= card dom f by FUNCT_4:99
    .= card f by Th60;
end;
