There are 16 cases.



Case 1. (allowed)

---->                              
                                       ---->
------------------------>              ------------------------>


Case 2. (disallowed)

---->                              
                                                           ---->
------------------------>              ------------------------>

Case 3. (disallowed)

---->                              
                                       <----
------------------------>              ------------------------>

Case 4. (allowed)

---->                              
                                                           <----
------------------------>              ------------------------>

Case 5. (disallowed)

                    ---->                              
                                       ---->
------------------------>              ------------------------>


Case 6. (allowed)

                    ---->                              
                                                           ---->
------------------------>              ------------------------>

Case 7. (allowed)

                    ---->                              
                                       <----
------------------------>              ------------------------>

Case 8. (disallowed)

                    ---->                              
                                                           <----
------------------------>              ------------------------>


Case 9. (disallowed)

<----
                                       ---->
------------------------>              ------------------------>


Case 10. (allowed)

<----
                                                           ---->
------------------------>              ------------------------>

Case 11. (allowed)

<----
                                       <----
------------------------>              ------------------------>

Case 12. (disallowed)

<----
                                                           <----
------------------------>              ------------------------>

Case 13. (allowed)

                    <----
                                       ---->
------------------------>              ------------------------>


Case 14. (disallowed)

                    <----
                                                           ---->
------------------------>              ------------------------>

Case 15. (disallowed)

                    <----
                                       <----
------------------------>              ------------------------>

Case 16. (allowed)

                    <----
                                                           <----
------------------------>              ------------------------>