fn encode_(
  dna: List(Nucleotide),
  acc: BitArray
) {
  case dna {
        [] -> acc
    [first, ..rest] ->
      {
        let nbit = encode_nucleotide(first)
        encode_(rest,
          <<acc:bits, nbit:size(2)>>)
    }
  }
}

pub fn encode(dna: List(Nucleotide)  )
  -> BitArray
{
  encode_(dna, <<>>)
}