decide equality.
Defined.

(* Specifications: *)
Theorem prop2 :
forall (a b : letter) (xs : list word),
a <> b ->
bar xs ->
forall ys : list word,
bar ys -> forall zs : list word, T a xs zs -> T b ys zs -> bar zs.
Theorem prop2 :
forall (a b : letter) (xs : list word),
a <> b ->
bar xs ->
forall ys : list word,
bar ys -> forall zs : list word, T a xs zs -> T b ys zs -> bar zs.