We will design some useful circuits using the basic logic gates, and use these circuits later on as building blocks for more complicated circuits.
We describe the basic AND, NAND, OR or NOR gates as being
satisfied when the inputs are such that a change in any one will
change the output.
A satisfied AND or NOR gate has a true output, whereas a satisfied
NAND or OR gate has a false output.
We sometimes identify the input logic variables A, B, C, etc.
with an n-bit number 
.