Basically the same way your favorite codec knows how to decompress. It builds the predictor (in this case, a NN) from the known input. The predictor then assign

Basically the same way your favorite codec knows how to decompress. It builds the predictor (in this case, a NN) from the known input. The predictor then assigns a symbol (or a series of symbols) to the next few bits - higher probabilities need fewer bits (I haven't checked for this specific technique, but there are techniques that don't even require an integer number of bits, e.g. range encoding).