[core] AddBefore/After/At option variant, as with Add/First/Last

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://its.cern.ch/jira/browse/ROOT-9288 (the first part)

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    18 files      18 suites   4d 11h 25m 49s ⏱️
 2 719 tests  2 719 ✅ 0 💤 0 ❌
47 253 runs  47 253 ✅ 0 💤 0 ❌

Results for commit 7132666.

[silverweed]

Do we really want to add support for a class that users are not supposed to ever use directly anymore? Is there a compelling current use case that requires this?

[ferdymercury]

Do we really want to add support for a class that users are not supposed to ever use directly anymore? Is there a compelling current use case that requires this?

Maybe it's better to ask the reporter of the JIRA ticket, who requested it, if he really needs it.
I am happy either way, if not, in that case https://its.cern.ch/jira/browse/ROOT-9288 can be closed as won't fix.

[silverweed]

Let's wait for @dpiparo and @pcanal's opinion on this.
The jira ticket is almost 7 years old at this point, so I doubt it's still relevant (but if we manage to contact the user we can hear what they have to say)