Hello, Rob Goldblatt in section 9.2 of his book "Topoi: The Categorical Analysis of Logic" introduces the notion of a "skeleton of a category C" which he defines as a "full subcategory C-sub-zero of C that is skeletal, and such that each C-object is isomorphic to one and only one C-sub-zero object". This statement seems to imply that we can have an "operator": skel: CAT -> CAT where CAT is the categories of (small) categories such that 1) skel is idempotent on any member of C of CAT, i.e. ] skel (skel (C)) = skel (C) 2) skel (C) = a "maximal" skeleton of C. I am struggling with 1) what "maximal" means in this case? E.g. is there some kind of order on all the skeletons of category C? 2) would the "operator" skel be a functor? Kind regards, Bill Halchin