This is not a mathematical question as such, but more about the correct grammatical way to describe the mathematical terms that are being introduced in a sentence.
I am used to writing sentences in this way:
A deterministic policy, $\pi$, is a function, $\pi: S \rightarrow A$, that selects an action, $a \in A$, for a state, $s \in S$.
But a reviewer for one of my paper submissions came back with a comment that I use too many commas around my variables. I used to think that my way is the correct way, since there is a pause when introducing the respective mathematical term.
I have seen papers that use either my approach, or a single comma after the term, or no commas at all.
What is the correct grammatical way to introduce mathematical terms in these kinds of sentences?
In English, appositives should generally be set off by commas: "Jack, my brother, stood six feet tall." rather than "Jack my brother stood six feet tall."
In mathematical writing, however, they're often omitted: "The indicator function $\chi_A$ for the set $A$ is defined as ..."
I personally don't like that choice, but your reviewer does. @Will Jagy has suggested a restructuring that may work in some situations if the lack of commas irritates you. And @Kurt G. has a good point: don't sweat the small stuff in reviews.