Gates: NOR
	NOR gate: implements NOR (negated OR) function
	Input: 2 bits
	Output: 1 bit
	Truth table:						Symbol:
		Input		Output
		x0	x1	z
		0	0	1
		0	1	0
		1	0	0
		1	1	0
			z = x0 NOR x1
	Properties:
	symmetric:		x NOR y = y NOR x
	not associative
	n inputs:
		NORn (x0, x1, . . . , xn) = NOT (x0 + x1 + . . . xn)