While gnu-gcc had `std::map:at const` as non-standard extension
it is n/a for older gcc on OSX.
Surprisingly this const& p() const; performs a tad better as well, likely
due to different exception handling.
Perhaps it is also worth investigating boost::flat_map<> as replacement
for std::map<>, here. Our common case is just a single entry, so using
a std::vector emulated mapping might help.
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