  Belnap-Dunn logic, also knows as the logic of First-Degree Entailment, is a
logic that can serve as the underlying logic of theories that are inconsistent
or incomplete. For various reasons, different expansions of Belnap-Dunn logic
with non-classical connectives have been studied. This paper investigates the
question whether those expansions are interdefinable with an expansion whose
connectives include only classical connectives. This is worth knowing because
it is difficult to say how close a logic with non-classical connectives is
related to classical logic. The notion of interdefinability of logics used is
based on a general notion of definability of a connective in a logic that seems
to have been forgotten.
