golint fails with:

Warning: exported: type name will be used as inserter.InserterFunc by other packages, and that stutters; consider calling this Func (revive)
Warning: exported: type name will be used as inserter.InserterFuncGenerator by other packages, and that stutters; consider calling this FuncGenerator (revive)

I think we probably shouldn't rename them as they're already public, but what do you think?

I believe both of those were only just added. We were planning on renaming them when we noticed the issue earlier this week. We haven't done a formal release of this module yet and I don't know if anyone other than you would be using them. Renaming them here or separately seems good.

Thanks @oschwald, I've renamed the funcs, added more test cases, and added the range-error check.

@oschwald Sorry to bump, but if you could review/merge this by today I'd be grateful: I need to use it somewhere ASAP 😄