https://www.boost.org/doc/libs/latest/doc/html/doxygen/structboost_1_1intrusive_1_1hash__key__equal.html
Struct template hash_key_equal
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https://www.boost.org/doc/libs/latest/doc/html/doxygen/structboost_1_1intrusive_1_1hash__key__types__base.html
Struct template hash_key_types_base
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