/**
  * Returns a map CitizenUniqueIdentifier => Citizen
  * @return array
  */
 protected function getCitizens()
 {
     $citizens = [];
     foreach ($this->population->getAllCitizenUniqueIdentifiers() as $identifier) {
         $citizens[$identifier] = $this->population->getCitizenByUniqueIdentifier($identifier);
     }
     return $citizens;
 }