Nesting Groups within Groups

I was able to nest a group within a group as well. I simply created 2 groups, then draged/droped one group into the other. However now I am unable to do so. Does anyone know if this is in fact a glitch?I was able to nest a group within a group as well. I simply created 2 groups, then draged/droped one group into the other. However now I am unable to do so. Does anyone know if this is in fact a glitch?

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