Finite-state adaptive block to variable-length noiseless coding of a nonstationary information source

The author associates with a given nonstationary finite-alphabet information source a certain class of stationary processes that he terms the stationary hull of the given source. He shows that the optimum average rate at which the given source can be noiselessly coded by means of a finite-state adaptive block to variable-length coding schemes is the largest entropy rate among those processes in the stationary hull. He explains what he means by an adaptive block to variable-length coding scheme