Indexing twice for conditional summation Clearly it is permitted to put restrictions on the indices in a summation, and

One often sees generalizations of this notation in which an arbitrary logical condition is supplied, and the sum is intended to be taken over all values satisfying the condition. Here are some common examples:
  $$\sum_{0\le k< 100} f(k)$$
  is the sum of $f(k)$ over all (integers) $k$ in the specified range,
  $$\sum_{x \mathop \in S} f(x)$$
  is the sum of $f(x)$ over all elements $x$ in the set $S$

However, if I don't have a symbol for the elements of my collection $E$, but rather address them as $E_i$, is it acceptable notation to index into the collection both in the condition and in the summand?
$$1\over\displaystyle\sum_{\substack{i=1 \\ E_i\neq 0}}^n\frac1{E_i}$$
If this is acceptable, it should give the same sum as this verbose use of Iverson brackets:
$$1\over\displaystyle\sum_{i=1}^n\frac{[E_i≠0]}{E_i+[E_i=0]}$$

I don't think it's a standard notation, but on the other hand I can hardly see how anybody could misinterpret that notation. One could use a more standardized notation, but that might suffer from it not being that easy to read, for example writing the entire condition below the sigma (ie $i\in\mathbb N\land 1\le i\le n\land E_i\ne 0$).
