Given a nested set $S = \{\{e_{11},e_{12},\dots,e_{1n}\},\{e_{21},e_{22},\dots,e_{2n}\},\dots,\{e_{k1},e_{k2},\dots,e_{kn}\}\}$, I want to specify a set consisting of specific subelements, for example $X = \{e_{12},e_{22},\dots,e_{k2}\}$. Therefore, it seems that a symbol for nested element would be helpful. For example:
$X = \{e_{i1}|e_{i1}\in\in S\}$ or $X = \{e_{i1}|e_{i1}\in^* S\}$
For single nesting I might use $X = \{e_{i1}|e_{i1}\in S'\in S\}$ or $X = \{e_{i1}|e_{i1} \in \bigcup S\}$ (which both do not look very nice), but what about nesting with an arbitrary number of levels?