(RULES
b a a -> a b c ,
c a -> a c ,
b c a -> a b c ,
c b -> d ,
a d -> d a ,
d -> b a , 
L a a -> L a b c ,
c R -> c b R
)
(COMMENT describes generalization of van Oostroms bean problem:
b a^{n+k+m} b ->* a^n b a^k b a^m  for n,m > 0, 
L = leftmost b, R = rightmost b)