def can_donate_blood_to(blood_group):
    """Return blood groups a user can donate to given a blood group."""
    can_donate_to = {
      'A+': ['A+', 'AB+'],
      'O+': ['O+', 'A+', 'B+', 'AB+'],
      'B+': ['B+', 'AB+'],
      'AB+': ['AB+'],
      'A-': ['A+', 'A-', 'AB+', 'AB-'],
      'O-': ['A+', 'O+', 'B+', 'AB+', 'A-', 'O-', 'B-', 'AB-'],
      'B-': ['B+', 'B-', 'AB+', 'AB-'],
      'AB-': ['AB+', 'AB-']
    }
    can_donate_blood_to = can_donate_to[blood_group]
    return can_donate_blood_to