  A general method of the BRST--anti-BRST symmetric conversion of second-class
constraints is presented. It yields a pair of commuting and nilpotent BRST-type
charges that can be naturally regarded as BRST and anti-BRST ones.
Interchanging the BRST and anti-BRST generators corresponds to a symmetry
between the original second-class constraints and the conversion variables,
which enter the formalism on equal footing.
