+ (iii.1) unit <=< p = p (here p has to be a natural transformation to M(1C))
+ (iii.2) p = p <=< unit (here p has to be a natural transformation from 1C)
+
+If `p` is a natural transformation from `P` to `M(1C)` and `q` is `(p Q')`, that is, a natural transformation from `PQ` to `MQ`, then we can extend (iii.1) as follows:
+
+ q = (p Q')
+ = ((unit <=< p) Q')
+ = ((join -v- (M unit) -v- p) Q')
+ = (join Q') -v- ((M unit) Q') -v- (p Q')
+ = (join Q') -v- (M (unit Q')) -v- q
+ ??
+ = (unit Q') <=< q