snt --> det(F), adjs(F), noun.
       det(F) --> [a], { F = consonant } ;
                   [an], { F = vowel }.
       adjs(F) --> adj(F); adj(F), adjs(_).
       adj(F) --> [spotted], { F = consonant } ;
                  [endangered], { F = vowel } ;
                  [poison_arrow], { F = consonant }.
       noun --> [frog] ; [owl].