#   a(n)b(n)c(n)d(n)
#   Fu, p. 94-95.
S->aAB
A->aAC
A->D
Dc->cD
Dd->dD
DC->EC
EC->Ed
DB->FB
Ed->Gd
cG->Gc
dG->Gd
aG->abcD
bG->bbcD
dFB->dFd
dFd->Fdd
cF->Fc
bF->bbc
aF->ab
bB->bcd
S:5