def can_receive_blood_from(blood_group):
    """Return allowed blood groups given a specific blood group."""
    can_receive_from = {
      'A+': ['A+', 'A-', 'O+', 'O-'],
      'O+': ['O+', 'O-'],
      'B+': ['B+', 'B-', 'O+', 'O-'],
      'AB+': ['A+', 'O+', 'B+', 'AB+', 'A-', 'O-', 'B-', 'AB-'],
      'A-': ['O-', 'A-'],
      'O-': ['O-'],
      'B-': ['B-', 'O-'],
      'AB-': ['AB-', 'A-', 'B-', 'O-']
    }
    can_receive_blood_from = can_receive_from[blood_group]
    return can_receive_blood_from