See the slides:
 http://ii.fmph.uniba.sk/cl/view/courses/2-INF-264-tdp/1516zs/pr/doc/handout.pr09.pdf
See also Sect. 2.3 pg(s). 22-24 in
 "J. Komara and P. J. Voda. Lecture Notes in Theory of Computability. 2001."
 http://ii.fmph.uniba.sk/cl/view/courses/2-INF-264-tdp/doc/comp2001.pdf