anchor.diff:                  let dc = Fun.HighestBit(abs v) + 1
                   let off = if v < 0 then (1 <<< dc) - 1 else 0
                   let v = uint16 (off + v)
  -                let huff = Huffman.dcChromInv.[dc]
  +                let huff = coder.dcChromaInv.[dc]
                   x.Write(huff)
                   x.Write(Codeword(byte dc, v))
   
  
positive.code:                 let v = uint16 (off + v)
  
                  let index = (byte leadingZeros <<< 4) ||| byte dc
                  let huff = Huffman.acLumInv.[int index]
                  x.Write huff
                  x.Write(Codeword(byte dc, v))
  
  
negative.code:                 Array.concat [
                      [| 0xFFuy; 0xDBuy; 0x00uy; 0x84uy |]
                      [| 0x00uy |]
                      QLum |> zigZag |> Array.map byte
                      [| 0x01uy |]
                      QChrom |> zigZag |> Array.map byte
                  ]
              ms.Write(quant, 0, quant.Length)
  
  