Should an iterator be valid after appending to a Glib::ustring?

[Mike Fleetwood <mike fleetwood googlemail com>]

Are iterators into a Glib::ustring still valid after the string is
modified?  Specifically appended to or modified at or beyond where the
iterator points.  I didn't obviously see this mentioned in the
documentation.

I though it might be valid because the iterator appears to be
implemented using a pointer difference type.  However the following
small test program fails with:
  **
  ERROR:test.cc:28:int main(): assertion failed: (linestart <= cursor)
  Aborted

Thanks,
Mike

--8<-- test.cc --8<--
#include <glib.h>
#include <glibmm/ustring.h>

int main()
{
    Glib::ustring buf;
    Glib::ustring::iterator linestart = buf.begin();
    Glib::ustring::iterator cursor = buf.begin();
    gunichar uc = 'u';
    buf.append(1, uc);
    cursor = buf.end();
    g_assert(buf.begin() <= linestart);
    g_assert(linestart <= cursor);
    g_assert(cursor <= buf.end());
    return 0;
}