  Our basic structure is a finite-dimensional complex Hilbert space $H$. We
point out that the set of effects on $H$ form a convex effect algebra. Although
the set of operators on $H$ also form a convex effect algebra, they have a more
detailed structure. W introduce sequential products of effect and operations.
Although these have already been studied, we introduce the new concept of
sequential products of effects with operations and operations with effects. We
then consider various special types of operations. After developing properties
of these concepts, the results are generalized to include observables and
instruments. In particular, sequential products of observables with instruments
and instruments with observables are developed. Finally, we consider
conditioning and coexistence of observables and instruments.
