Section 9.3: Redraw the circuit of problem 9.3.5, using only NAND
gates, NOR gates, and inverters. Follow the "bubble convention",
which puts inversion bubbles at both ends of a wire (an active-low
signal) or at neither end of the wire (an active high signal).
8 (hint: you can make a much smaller circuit if you first design an
XOR module that computes the exclusive-or of two signals),15,16,18