Solve "Out of local stack" in this specific constraint programming in prolog - variables

Question

I'm currently trying to create schedules for bus drivers in prolog. I wish to find a limited number of solutions. But I get the "Out of local stack" error, and I suppose it is because I'm getting too many solutions.
How can I prevent that error given the following code? Any tips on whatever I'm not doing correctly would help immensely too.
count_drivers: counts the number of drivers with D_id as driver_id
( I need them to work less than "max_hours").
vehicle: represents the bus and respective routes.
connected: represents the connection between the relief opportunities
( a route consists of a group of relief points and the respective "connection"
between them)
workpiece: is a segment of work in the same vehicle between two relief points
spell: is a group of workpieces done by the same driver
spreadover: is the whole shift one driver has to do.
Here is the code:
?- use_module(library(clpfd)).
?- use_module(library(lists)).
?- use_module(library(aggregate)).
%workpiece(Bus,[Ro1,Ro2],Weight).
workpiece(1,[1,2],1).
workpiece(1,[2,3],2).
workpiece(1,[3,4],1).
workpiece(1,[4,5],2).
workpiece(1,[5,6],1).
workpiece(2,[7,8],2).
workpiece(2,[8,9],2).
workpiece(2,[9,10],1).
workpiece(2,[10,11],2).
workpiece(2,[11,12],1).
workpiece(3,[13,14],2).
workpiece(3,[14,15],1).
workpiece(3,[15,16],2).
workpiece(3,[16,17],1).
workpiece(3,[17,18],2).
%spell
spell(Vehicle,[[Ro1,Ro2]|Tail]):-Vars = [Ro1,Ro2], Vars in 1..18, workpiece(Vehicle,[Ro1,Ro2],_),spell(Vehicle,Tail,Ro2),labeling([],Vars).
spell(_,[],_).
spell(Vehicle,[[Ro1,Ro2]|Tail],Ro3):- Vars = [Ro3], Vars in 1..18, Ro3 #= Ro1, workpiece(Vehicle,[Ro1,Ro2],_),spell(Vehicle,Tail,Ro2), labeling([],Vars).
%spreadover de cada driver
spreadover(_,List):- Vars = I, Vars in 1..15, length(List,I), I #>= 1.
spreadover(Driver,[Head|Tail]):- Vars = [Vehicle,I], Vars in 1..9, Vehicle #>= 1, Vehicle #=< 3, spell(Vehicle,Head), length(Head,I), I #>= 1, spreadover(Driver,Tail), labeling([],Vars).
%ocupar as workpieces todas
%minimizando os shifts
%cobrir todas as routes
%length 15
%drivershifts
drivershifts(_,List):- Vars = I, Vars in 1..15, length(List,I), I #= 15.
drivershifts(NumDrivers,[[Driver|List]|Tail]):-Vars = Driver, Vars in 1..NumDrivers, Driver #>= 1, Driver #=< NumDrivers, spreadover(Driver,List), labeling([],Vars).
I thank you all in advance for any time you can spare in helping me.
EDIT: I changed the code around a bit, now I get a load of unassigned variables from a query of
forall(spreadover(1,List),writeln(List)).
or one unassigned variable from
spreadover(1,List).
I restricted the domains wherever I could, but aren't sure if I'm doing this correctly.
From the queries above I should generate spreadovers(a set of spells) for driver 1.
Not sure if I should post a new question or rewrite this one either, so decided to rewrite this one.

Answer 1

You have many warnings from Singleton variables, and would be good style to solve them.
At least, prefix variables you know are unused with an underscore, to avoid the warning.
Now to the loop: you're calling diagram with a free variable, causing an infinite recursion that 'construct' an infinite list of partially instantiated variables.
I can't understand what could be the intended meaning of diagram/1. For sure, you miss the base case: add something like
diagram([]).