(1) a b c d  (ab + c)  (a + b' + d)  (a'c + bd)  (a' + b' + c')
    -------  --------  ------------  ----------  --------------
    0 0 0 0      0        1              0           1
    0 0 0 1      0        1              0           1
    0 0 1 0      1        1              1           1
    0 0 1 1      1        1              1           1
    0 1 0 0      0        0              0           1
    0 1 0 1      0        1              1           1
    0 1 1 0      1        0              1           1
    0 1 1 1      1        1              1           1
    1 0 0 0      0        1              0           1
    1 0 0 1      0        1              0           1
    1 0 1 0      1        1              0           1
    1 0 1 1      1        1              0           1
    1 1 0 0      1        1              0           1
    1 1 0 1      1        1              1           1
    1 1 1 0      1        1              0           0
    1 1 1 1      1        1              1           0


(2) ab\cd | 00 01 11 10
    ------+------------
    00    |  1  1  1  1
    01    |  1  1  1  1
    11    |  1  1  0  0
    10    |  1  1  1  1


(3) any one of the following works:
        ab'd' + cd' + a'b'd + abd + bc'd
        ab'd' + cd' + a'b'd + abd + a'c'd
        ab'd' + cd' + a'b'd + bc'd + abc
        ab'd' + cd' + a'b'c + a'c'd + abd

(4) see lab2.circ file