  Reversible computing is a paradigm of computation that reflects physical
reversibility, one of the fundamental microscopic laws of Nature. In this
survey, we discuss topics on reversible logic elements with memory (RLEM),
which can be used to build reversible computing systems, and their
universality. An RLEM is called universal, if any reversible sequential machine
(RSM) can be realized as a circuit composed only of it. Since a finite-state
control and a tape cell of a reversible Turing machine (RTM) are formalized as
RSMs, any RTM can be constructed from a universal RLEM. Here, we investigate
2-state RLEMs, and show that infinitely many kinds of non-degenerate RLEMs are
all universal besides only four exceptions. Non-universality of these
exceptional RLEMs is also argued.
