equivalent expression: (~a|~b|~c)
c,      b,      a,      out
0,      0,      0       = 1
0,      1,      0       = 1
0,      0,      1       = 1
0,      1,      1       = 1
1,      0,      0       = 1
1,      1,      0       = 1
1,      0,      1       = 1
1,      1,      1       = 0