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I was wondering about this expression 入っている for something being inside. I always wondered about it, since it literally should mean being in the process of entering.. It would make more sense to me if it were 入ってある (like おいてある or 張ってある), but I'm aware that this construction only works for 他動詞, so it maybe should be 入れてある..? Does anyone know how this odd structure 入っている came about?