DEFINITION arity_gen_cvoid()
TYPE =
       ∀g:G
         .∀c:C
           .∀t:T
             .∀a:A
               .arity g c t a
                 →∀d:C
                      .∀u:T
                        .∀i:nat
                          .getl i c (CHead d (Bind Void) u)
                            →ex T λv:T.eq T t (lift (S O) i v)
BODY =
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