CS1950Y Lecture #8: Cities
Date 2/8/2019

    // Feb 8th
    sig City {
      roads: set City
    }
    one sig Pvd extends City {}
    one sig Bos extends City {}
    one sig Wor extends City {}
    //pred noSelfLoops {
    fact noSelfLoops {
      //Pvd not in Pvd.roads
      all c: City | c not in c.roads
    //  no c: City | c in c.roads
    // one c: City | c in c.roads
    }
    //pred anotherNoSelfLoops {
    //  no iden & roads
    //}
    run {} for exactly 5 City
    //run noSelfLoops for exactly 5 City