a is absent from 
b is absent from a
a is absent from c
bb is absent from bcabacbaa
aac is absent from aaabacbbcacc
aaca is absent from aaabbbcccaaacccbbbbbbaaacccbbbcccaaacccaaabbbcccbbbaaaaabbccaaccbbbbaaccbbccaaccaabbccbbaaabcacbbacbcacabcbaaaabacbabbbccacbcc
aaac is absent from cccacbbacabacabacababababacaacacababcabcabcaacababcacaabacbcabcacacacacabaabacabaabbccaaaabbbcaacacaaabbbbbcaccbcbaba
ac is absent from aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbcccccccccccccccccccccccccccccccccccccccccccccccccc
a is absent from cb
a is absent from 
c is absent from ab