asset class Prescription {…}

interface Pharmacy {
  void depositPrescription(owned Prescription >> unowned Prescription p);  
  owned Prescription removePrescription(Prescription @ Unowned p);
  unowned Prescription getAnyPrescription();
}
 
class Patient {
  void fillPrescription(unowned Pharmacy pharmacy, unowned Prescription pre) {
    pharmacy.depositPrescription(pre); // Location (A)    
    
    unowned Prescription pre2 = pharmacy.getAnyPrescription();
    owned Prescription removedPrescription = pharmacy.removePrescription(pre2); // Location (B)
    pharmacy.depositPrescription(removedPrescription); // Location (C)
  }
}

For each location where the compiler will give an error, check the corresponding box and explain why there is an error.