 As we said, our work here begins to use category theory. We realize that not all readers will be familiar with that subject at all.     So we shall  try to make this section as accessible as possible. In particular, we’ll  only present those notions from category theory that we actually need in our work of this section.      We also illustrate all of the  definitions on a few categories which will be of interest.  And as we go on in future sections, we’ll develop only the background that we  need.[11] 