Early implementations of integer_sequence used a naive implementation
that required one level of template depth per additional integer in the
sequence. Rebirth uses a private alternate implementation named
make_tree_index_sequence that requires only log(N) steps for an
N-element index_sequence. Recent versions of gcc ship a log(N) version
of integer_sequence. Probe for that version and, if found, use it
instead of the private implementation, on the theory that the compiler
writers did at least as good a job as I did, and possibly better if they
were able to leverage compiler implementation details.
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