REACTION OF HYDROGEN FLUORIDE IN PYRIDINE SOLUTION WITH CIS-CYANO-2 AND CIS-AMIDO-2-AZIRIDINES. PREPARATION OF β-FLUORO α-AMINO ACIDS AND ESTERS BY MEANS OF ACID HYDROLYSIS AND ALCOHOLYSIS OF β-FLUORO-α-AMINONITRILES AND/OR β-FLUORO-α-AMINO ACID AMIDES