title: How to add variable on a set (Alloy)
body: <pre><code>module carona
  
  open util/ordering[Time]
  
  sig Time {}
  
  sig Car {
      passengers : set Passager
  }
  
  one sig UFCG{
      students: set Student -&gt; Time
  
  }
  
  abstract sig Student{
      region: Region -&gt; Time
  }
  
  sig Owner extends Student{
      ownerCar : Car 
  }
  
  sig Passager extends Student{
  }
  
  abstract sig Region{ }
  
  one sig Norte,Sul,Leste,Oeste,Centro extends Region{ }
  
  
  
  fact Owner{
      all o: Owner | #o.ownerCar = 1
  }
  
  fact Passager{
      all p:Passager | one p.~passengers  
  }
  
  fact Car{
      all c:Car| one c.~ownerCar
      all c:Car| #c.passengers &lt;= 4
      all c:Car| c.passengers.region=(c.~ownerCar).region 
  
  }
  
  fact UFCG {
      all passager: Passager, ufcg : UFCG, t:Time | passager in (ufcg.students).t
      all owner: Owner, ufcg: UFCG,t:Time | owner in (ufcg.students).t
  } 
  
  fact Traces{
      init[first]
      all pre: Time-last | let pos = pre.next |
      lone u:UFCG|
      some s:Student|
      addStudent[s,u,pre,pos]
  }
  
  
  
  pred init[t:Time]{
      one u:UFCG | no (u.students).t 
  }
  
  
  
  pred addStudent[s:Student, u: UFCG, before, after: Time]{
  
      (s !in (u.students).before) 
      (u.students).after = (u.students).before + s
  }
  </code></pre>
  
  <p>After I run and show the instances created over time, my pred to add Passenger doesn't seem to work, nothing changes or even create. How do i properly add items on a set?</p>
  
  <p>My program, when run, does not show any instance. That is, I can not add anything over time.</p>