Loop of length 20 starting with a string of length 13
found after enumerating 1914 derivations:

.aaaa.bbaabbbba
	rule aaaa -> bbba at position 0
.bbba.bbaabbbba
	rule babb -> abba at position 2
.bbabba.aabbbba
	rule babb -> abba at position 1
.babbaa.aabbbba
	rule babb -> abba at position 0
.abbaaa.aabbbba
	rule aaaa -> bbba at position 4
.abbabbba.bbbba
	rule babb -> abba at position 2
.ababbaba.bbbba
	rule babb -> abba at position 1
.aabbaaba.bbbba
	rule babb -> abba at position 6
.aabbaaabba.bba
	rule babb -> abba at position 8
.aabbaaababba.a
	rule babb -> abba at position 7
.aabbaaaabbaa.a
	rule aaaa -> bbba at position 4
.aabbbbbabbaa.a
	rule babb -> abba at position 6
.aabbbbabbaaa.a
	rule babb -> abba at position 5
.aabbbabbaaaa.a
	rule babb -> abba at position 4
.aabbabbaaaaa.a
	rule babb -> abba at position 3
.aababbaaaaaa.a
	rule babb -> abba at position 2
.aaabbaaaaaaa.a
	rule aaaa -> bbba at position 6
.aaabbabbbaaa.a
	rule babb -> abba at position 4
.aaababbabaaa.a
	rule babb -> abba at position 3
.aaaabbaabaaa.a
	rule aaaa -> bbba at position 9
.aaaabbaabbbba.