[0] ++ sendSeq ++ recvSeq ++ [poll]

say sendSeq was one bit but we still wanted it to take 3 bits:

[0] ++ pad 3 sendSeq ++ poll ++ recvSeq

given some "pad" function defined in a library

The only reason it's "hard" in C is because the operations are induced from what the machine can do efficiently and without "a sufficiently smart compiler".

    struct build_info_control
    {
      unsigned char         : 1;
      unsigned char SendSeq : 3;
      unsigned char Poll    : 1;
      unsigned char RecvSeq : 3;
    };

    struct build_info_control control;

    control.SendSeq = sendSeq;
    control.Poll    = true;
    control.RecvSeq = recvSeq;

> say sendSeq was one bit but we still wanted it to take 3 bits:

> [0] ++ pad 3 sendSeq ++ poll ++ recvSeq

In Erlang you could do this to get something padded (or truncated) to the correct size:

  << 2:3 >> ;; corresponds to the 3 bits <<010>>

  << 0:1, SendSeq:3, Poll:3, RecvSeq:3>>

The entire language is built around pattern matching to the point where it simply wouldn't exist otherwise, and I assume binary protocol handling was a core requirement when the language was designed since it was created for telecom switches.