Confusions in Iterator-View.md

[BenWeber42]

After reading about the iterator view there were some confusions left which I want to mention here.
https://github.com/GridTools/concepts/blob/master/Iterator-View.md

The explanation following this paragraph is quite confusing to me:

If all the builtins are defined that way, we can get rid of using lift and deref in the stencil composition. That is probably a good idea for the frontend, but not for the intermediate level. Here the illustration. Suppose we have a stencil:

Most of the functions used there aren't defined anywhere (e.g., hannes_sum).

A bit subjective, but I find the EBNF notation a bit hard to read (maybe use space instead of comma for concatenation and use ( ... )* for repetition instead of { ... }).

Then some of the signatures are a bit confusing, e.g. what does this mean:

Stencil<Optional<V>, V> deref;