Necessary laws

For ◎ to work like composition, we need join to hold the following properties:

• join (join (F (F (F a))))=join (F (join (F (F a))))
• abusing notations denoting join by ⊙; this is equivalent to
  (F ⊙ F) ⊙ F = F ⊙ (F ⊙ F)
• There exists η :: a -> F a s.t.
  η⊙F=F=F⊙η