[erlang-questions 7] Why bit syntax matching can't be used in match_spec?

[max tan]

It seems bit syntax matching can't be used in match_spec, I just wonder why.

in erlang shell:
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39> mnesia:dirty_select(main_dir, [{{k_v, <<"/root"/binary, "_"/binary>>,
'_'}, [], ['$_']}]).
** exception error: bad argument

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47> ets:fun2ms(fun ({k_v, <<"/root", Res/binary>>, _}) -> Res end).
Error: fun head contains bit syntax matching of variable 'Res', which cannot
be translated into match_spec
{error,transform_error}
48>

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