Urquhart and Mendez and Salto claim to establish completeness theorems for
the system C and two of its negation extensions. In this note, we do that f
ollowing three things: (1) provide a counterexample to all of these alleged
completeness theorems, (2) attempt to diagnose the mistakes in the reporte
d completeness proofs, and (3) provide complete axiomatizations of the desi
red systems.