Section 3.6 Reference Topics
This site considers logic gates (with links to other logic topics):
http://www.phys.ualberta.ca/~gingrich/phys395/notes/node122.html
This site is more advances than most we recommend, but it can be used to show how the circuit
designs of this section can actually be utilized to design a calculator:
http://www.cs.rochester.edu/u/leblanc/csc173/proplogic/circuits.html