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I want to insert something into a STL list in C++, but I only have a reverse iterator. What is the usual way to accomplish this?

This works: (of course it does)

std::list<int> l;
std::list<int>::iterator forward = l.begin();
l.insert(forward, 5);
This doesn't work: (what should I do instead?)
std::list<int> l;
std::list<int>::reverse_iterator reverse = l.rbegin();
l.insert(reverse, 10);
Essentially, you don't.  See 19.2.5 in TCPPPL.
The reverse_iterator has a member called base() which will return a "regular" iterator.  So the following code would work in your example:
l.insert(reverse.base(), 10);
Be careful though because the base() method returns the element one after the orginal reverse_iterator had pointed to.  (This is so that reverse_iterators pointing at rbegin() and rend() work correctly.)
Just in case it is helpful, as this it the first hit on a search, here is a working example of using rbegin. This should be faster than using std::stringstream or sprint. I defiantly call a member fmt_currency many thousands of times in some print jobs. The std::isalnum is for handling a minus sign.
std::wstring fmt_long(long val) {//for now, no options? Just insert commas
    std::wstring str(std::to_wstring(val));
    std::size_t pos{ 0 };
    for (auto r = rbegin(str) + 1; r != str.rend() && std::isalnum(*r); ++r) {
        if (!(++pos % 3)) {
            r = std::make_reverse_iterator(str.insert(r.base(), L','));
    return str;
        
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