Dear all, just an observation. The facts List.drop_Suc_conv_tl: ?i < length ?xs ⟹ ?xs ! ?i # drop (Suc ?i) ?xs = drop ?i ?xs List.nth_drop': ?i < length ?xs ⟹ ?xs ! ?i # drop (Suc ?i) ?xs = drop ?i ?xs are duplicates of each other. cheers chris