anchor.diff:          revTailLenMax
           (fwd:Primer)
           (rev:Primer)
  -        (middleDNA : char array) =
  +        (middleDNA : Dna) =
       // Input is two sets of primers like this where the region between | symbols is an inline sequence.
       // Output is adjust primer tails that have a better annealing length
   
  
positive.code:                     match sandwichR,sandwichF with
                      | None,None -> revArg, hd.dna, fwdArg, 0,0
                      | Some(r),None ->
                         ({revArg with tail = Array.concat [r.dna ; hd.dna] |> revComp},
                          Array.concat [r.dna; hd.dna],
                          fwdArg,
                          r.dna.Length,
                          0)
                      | None,Some(f) ->
                         (revArg,Array.concat [hd.dna; f.dna],
                          {fwdArg with tail = Array.concat [hd.dna;f.dna]},
                          0,
                          f.dna.Length)
                      | Some(r),Some(f) ->
                         ({revArg with tail = Array.concat [r.dna; hd.dna] |> revComp},
                          Array.concat [r.dna; hd.dna; f.dna],
                          {fwdArg with tail = Array.concat [hd.dna; d.dna]},
                          r.dna.Length,
                          f.dna.Length)
  
  
negative.code:             let x =
                  if next.dna.Length < 10 then
                      if verbose then
                          printf "WARNING: template dna (%s) (len=%d) %O too short for succcessful primer design\n"
                              next.description next.dna.Length next.dna
                      None
                  else
                      if next.sourceFrApprox then
                          let task =
                             {tag ="PF";
                              temp = next.dna.[0..min (next.dna.Length-1) margin].arr;
                              align = ANCHOR.CENTERLEFT;
                              strand = STRAND.TOP;
                              offset =0;
  