put together statements about `]x-r,x+r[`

This was a remark made by @zstone1 in the conversation about PR #973 .

"Much of the results we've made will generalize to borel measures, (but without HB it's a bit of a nightmare to try to construct the type of measure + topological space). Factoring out results that we know will be generalizable might be nice to give us a place to start in the future."
