“…The string coordinates are called 'matter' fields while the non-decoupled gravitational fields are represented by a set of so-called 'Liouville' fields. In [9,41], the BRST operators for the Liouville system were obtained. Especially, in [37,42], the BRST operators of W M 2,s ⊗W L 2,s ′ were constructed at the classical level, and much valuable results were given.…”