http://3cbzkrvakrpetjjppdwzbzqrlkmzatjs7jbyazap5gwutj32gcltjpqd.onion
So for instance, assuming that x and y are e.g. positive integers
(including zero), then max(x,y) (the motivating example for this post)
is a monoid. Why? Because it's a non-crazy binary operation
(in particular, max(max(x,y),z) = max(x,max(y,z))),
and you can use x=0 or y=0 as a no-op (max(anything, 0) = anything).