See the slides:
 http://ii.fmph.uniba.sk/cl/view/courses/1-AIN-625-lpp/1314zs/pr/doc/handout.pr08.pdf
See also Sect. 5.1 pg(s). 83-84 in 
 "J. Komara and P. J. Voda. Lecture Notes in Theory of Computability. 2001."
 http://ii.fmph.uniba.sk/cl/view/courses/1-AIN-625-lpp/doc/comp2001.pdf