We limit ourselves to recalling some basic definitions. A category is called an Ab-category if it is enriched over (the symmetric tensor category Ab of) abelian groups. I.e., all hom-sets come with abelian group structures and the composition • of morphisms is a homomorphism w.r.t. both arguments. An Ab-category is called additive if it has a zero object and every pair of objects has a direct sum. If k is a field (or more generally, a commutative unital ring) then a category C is called k-linear if all hom-sets are k-vector spaces (or k-modules) and • is bilinear. An object X is called simple if every monic morphism Y ֒→ X is an isomorphism and absolutely simple if End X ∼ = kid X . If k is an algebraically closed field, as we will mostly assume, the two notions coincide. A k-linear category is semisimple if every object is a finite direct sum of simple objects. A semisimple category is called finite if the number of isomorphism classes of simple objects is finite. (There is a notion of finiteness for non-semisimple categories, cf.[34], but we will not need it.) A positive * -operation on a C-linear category C is a contravariant endofunctor of C that acts like the identity on objects, is involutive ( * * = id) and anti-linear.(Dropping the C-linearity, one arrives at the notion of a dagger-category.) A * -operation is called positive if s * • s = 0 implies s = 0. A category equipped with a (positive) * -operation is called hermitian (unitary). A unitary category with finite-dimensional hom-spaces and splitting idempotents is semisimple (since finite dimensional algebras with positive * -operation are semisimple, thus multi-matrix algebras).