DEFINITION csuba_drop_abbr()
TYPE =
       ∀i:nat
         .∀c1:C
           .∀d1:C
             .∀u:T
               .drop i O c1 (CHead d1 (Bind Abbr) u)
                 →∀g:G
                      .∀c2:C
                        .csuba g c1 c2
                          →ex2 C λd2:C.drop i O c2 (CHead d2 (Bind Abbr) u) λd2:C.csuba g d1 d2
BODY =
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