1. MacLane, S., Birkhoff, G.: Algebra. New York: MacMillan 1967
2. Munkres, J. R.: Elementary linear algebra. Reading (Mass.): Addison-Wesley 1964
3. Floyd, R.: Assigning meanings to programs. In: Schwartz, J. (ed.): Mathematical aspects of computer science 19. Providence (R.I.): American Mathematical Society 1967, p. 19?32
4. Wegbreit, Ben: Property extraction in well-Founded property sets. Center for Research in Computing Technology, Harvard University, Cambridge (Mass.) and Computer Science Division, Bolt, Beranek, and Newman, Inc., Cambridge (Mass.), February 1973
5. Karr, M.: Gathering information about Programs. Massachusetts Computer Associates, Inc., (In preparation)