by Zsolt Laszlo Kovacs.Bibliography: p. 175-184.Originally presented as the author's thesis, (Ph.D.) in the M.I.T. Dept. of Electrical Engineering and Computer Science, 1977. Partial support provided by the Funda? de Amparo a Pesquisa do Estado de S?Paulo, Brazil and by the National Science Foundation Grant no. NSF/ENG-7705200.