Solution
%% An abcd string can consist of only a's and b's
abcd --> bc.
%% An abcd string can consist of an a followed by an abcd string
%% followed by a d
abcd --> onea, abcd, oned.
%% A bc string can be empty.
bc --> [].
%% Otherwise, it consists of two b's followed by a bc string followed
%% by two c's.
bc --> oneb, oneb, bc, onec, onec.
onea --> [a].
oneb --> [b].
onec --> [c].
oned --> [d].