% Startsymbole:

start_symbol(s(pl)).
start_symbol(s(sg)).

% Grammatikregeln:

:- op(1100,xfx,'--->').

s(Num) ---> a(sg), 
        ( b(sg), {Num = sg} 
        ; b(pl), {Num = pl}
        ).
b(pl) ---> b(sg), ([c], b(pl) ; a(sg)).

% Lexikon:

word(a,a(sg)).
word(b,b(sg)).