Ошибка: "Free variable in expression" - Prolog

Формулировка задачи:

Друзья, помогите побороть эту ошибку: "Free varable in expression" в выражении scan(V,Q):-V<=Q,

                    Листинг программы
                
 
                            
                        
nowarnings
domains
    q = string
    n = integer
predicates 
payroll (n,q,n,n,q,n) 
data_bank   
scan(n,n)   
process(n) 
repeat      
do_mbase   
menu        
error       
goal 
do_mbase.
 
clauses 
 
payroll (1,"24.01.2016",6,60,"mostly cloudy",754).
payroll (2,"25.01.2016",2,75,"snow to rain",764).
payroll (3,"26.01.2016",0,60,"partly cloudy",754).
repeat. 
repeat:-repeat. 
 
do_mbase:-makewindow(1,7,7,"Menu",0,0,25,80),menu. 
menu:-repeat, clearwindow,
    write("Select the menu item: "), nl, nl, 
    write("1. View the database"), nl ,
    write("2. Пошук запису по прізвищу працівника"), nl, 
    write("3. Знайти всіх робітників чий оклад не перевищуЇ суму, що вводиться"), nl, 
    write("0. Exit"), nl ,
    write("********"), nl, nl ,
    write("Enter the menu item"), nl ,
readint(X),nl,
process(X),X=0,!. 
 
process (1):- 
makewindow (2,7,7,"The database",2,20,18,58),clearwindow, 
write ("Database :  "),
            cursor (3,14),data_bank,nl,nl,
                write("Press any key"),readchar(_),fail.
 
process (2):- 
makewindow(3,14,14,"Пошук",0,0,20,40), 
write("Search by description of the weather: "), 
write("Enter description "), readln(Z),
payroll(X,Y,V,B,Z,C),  cursor(7,14),                                                             
                       write("Number - ",X),nl,       
                       write("Date - ",Y),nl,         
                       write("Temperature - ",V),nl,   
                       write("Humidity - ",B),nl,     
                       write("Description - ",Z),     
                       write("Pressure - ",C),nl,nl,nl, % Атм. тиск
                       write("Press any key"),readchar(_),      
removewindow.
 
/* Перегляд інформації з БД */
process (3):-makewindow (4,7,7,"Window 3",0,0,24,70), 
write ("The temperature is below of: "),               
write ("Еnter temperature"), readint(Q), 
             payroll(_,_,_,_,_,_), 
scan(V,Q).
 
/* Вихід з діалогу */
process (0):- 
             write("Press any key"), 
             readchar(_),exit.
process (X):-X<0,error, 
             X>3,error. 
    
error:- write ("The number must be 0-3"),nl,nl,
                write ("Press any key"),readchar(_).
                
data_bank:-             
payroll(X,Y,V,B,Z,C),nl,          
                   write("Number - ",X),nl,       
                   write("Date - ",Y),nl,          
                   write("Temperature - ",V),nl,  
                   write("Humidity - ",B),nl,      
                   write("Description - ",Z),     
                   write("Pressure - ",C),nl,nl,  
                   write("Press any key"), readchar(_),nl,fail.
data_bank :- !.
 
scan(V,Q):-V<=Q,  
                   write("Number - ",X),nl,      
                   write("Date - ",Y),nl,         
                   write("Temperature - ",V),nl,  
                   write("Humidity - ",B),nl,      
                   write("Description - ",Z),     
                   write("Pressure - ",C),nl,nl,  
                   write("Press any key"), readchar(_),nl,fail.                      
scan(V,Q):-!.

Решение задачи: «Ошибка: "Free variable in expression"»

textual
                            Листинг программы 
                        
write ("Еnter temperature"), readint(Q), 
             payroll(_,_,_,_,_,_), 
scan(V,Q).