[erlang-questions] confused by the grammar of split_binary/2 in the example of mp3_sync

[lang qiu]

Hi,

I am confused by the grammar of split_binary/2 in the example of mp3_sync in
the book "Programming Erlang",

get_word(N, Bin) ->
   * {_,<<C:4/binary,_/binary>>} = split_binary(Bin, N), C.*

I understand that *@spec split_binary(Bin, Pos)* -> {Bin1, Bin2} but how
does the above clause work ? How does it make C get the first 4 bytes of Bin
starting from N ?

Thanks!
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