These are chat archives for boostorg/hana
keys(map) == unpack(map, make_tuple ^on^ first)
keys(map, fn) == unpack(map, fn ^on^ first)
keys(map, forward_as_tuple)and they explicitly ask for a container of references
filtercould also be made to call an arbitrary function object with the filtered elements instead of returning a sequence. This is the idea behind the Fit library.