The transfusion checker

A dictionary gives the antigens on the red cells of each ABO group. You will work out the antibodies, decide whether a transfusion is safe, and list the donors for a patient.

The idea. A person has the antibody against every antigen that his own cells lack. A transfusion is safe when no antigen of the donor meets an antibody of the recipient.

Your tasks.

1. Finish antibodies. It gives the set of antibodies of a group.
2. Finish safe. It gives True if the donor can give to the recipient.
3. Finish donors_for. It gives the list of safe donor groups for a recipient, in the order A, B, AB, O.