In a formal treatment one would clear up any possible ambiguity
that arises in the use of a term such as this. To me, it's a
simple matter of writing
a + a + a + ... + a
Indexing also shows up in board games, like do you start counting on the square you are on, or the next one. I remember my son's confusion (when he was 5). You actually NEED this ambiguity early on in order to distinguish differences in context. It is a critical and essential part of higher level reasoning that should be embraced.