"reject". Consequently, every NP-complete problem can be converted to a co-NP complete problem.

This does not mean, however, that the concept of co-NP-completeness is useless, since it is possible that there exists a problem that is NP but not co-NP. Reversing the labels "accept" and "reject" results in a problem that is co-NP but not NP. It is an open problem whether NP=co-NP. Since the problem of P=NP is also open, all known instances of problems in are stuck in '''P'''.