Loop of length 21 starting with a string of length 6
found after enumerating 5312 derivations:

.cb.abba
	rule cb -> aacc at position 0
.aacc.abba
	rule ca -> bb at position 3
.aacbb.bba
	rule cb -> aacc at position 2
.aaaaccb.bba
	rule cb -> aacc at position 5
.aaaacaacc.bba
	rule cb -> aacc at position 8
.aaaacaacaacc.ba
	rule ca -> bb at position 7
.aaaacaabbacc.ba
	rule ab ->  at position 6
.aaaacabacc.ba
	rule ab ->  at position 5
.aaaacacc.ba
	rule cb -> aacc at position 7
.aaaacacaacc.a
	rule ca -> bb at position 6
.aaaacabbacc.a
	rule ab ->  at position 5
.aaaacbacc.a
	rule cb -> aacc at position 4
.aaaaaaccacc.a
	rule ca -> bb at position 7
.aaaaaacbbcc.a
	rule cb -> aacc at position 6
.aaaaaaaaccbcc.a
	rule cb -> aacc at position 9
.aaaaaaaacaacccc.a
	rule ca -> bb at position 14
.aaaaaaaacaacccbb.
	rule cb -> aacc at position 13
.aaaaaaaacaaccaaccb.
	rule ca -> bb at position 12
.aaaaaaaacaacbbaccb.
	rule cb -> aacc at position 11
.aaaaaaaacaaaaccbaccb.
	rule cb -> aacc at position 18
.aaaaaaaacaaaaccbacaacc.
	rule ca -> bb at position 17
.aaaaaaaacaaaaccbabbacc.