+ 4 A receives ADD_HOST(E) from B, and notes that it is already known:
+ <insert solution here>
+ A receives ADD_HOST(F) from B, and notes that it is already known:
+ <insert solution here>
+ F receives ADD_HOST(A) from E, and notes that it is already known:
+ <insert solution here>
+ F receives ADD_HOST(B) from E, and notes that it is already known:
+ <insert solution here>