a(S,S2)	:-concat([S, 'a'], S2).
b(S,S2)	:-concat([S, 'b'], S2).
c(S,S2)	:-concat([S, 'c'], S2).
s:-s(S), write(S), nl.
t:-t(T), write(T), nl.
u:-u(U), write(U), nl.
s(S):-s('', S).
s(S0,S):-c(S0,S) ;  a(S0,S1),s(S1,S2),b(S2,S).
t(T):-t('',T).
t(T0,T):-a(T0,T1), c(T1,T) ;  a(T0,T1),t(T1,T).
u(U):-u('',U).
u(U0,U):-c(U0,U) ;  a(U0,U1),b(U1,U2),b(U2,U3),u(U3,U) ;  a(U0,U1),b(U1,U2),u(U2,U3),b(U3,U) ;  a(U0,U1),u(U1,U2),b(U2,U3),b(U3,U) ;  u(U0,U1),a(U1,U2),b(U1,U2),b(U3,U).
:- write('Input s(S), nl? , t(T)?, or u(U)? to generate strings').
:- write('  But prolog won't generate all the U strings....'), nl.