1. A method for factoring an input finite-state transducer (FST) including an unknown symbol, comprising the steps of: replacing each occurrence of the unknown symbol in the input FST with the unknown symbol and a diacritic to define a left-sequential finite-state transducer (FST); and replacing each occurrence of the diacritic with a symbol representative of an empty string and an output symbol to define a right-sequential finite-state transducer (FST); wherein said replacing steps avoid direct factorization of the unknown symbol.