let rec fastq_sample_step ~compiler (fs : fastq_sample pipeline) =
    let {work_dir; machine ; _ } = compiler in

      match fs with
      | Bam_to_fastq (info_opt, how, what) ->
        let bam = compile_aligner_step ~compiler what in
        let sample_type =
          match how with `Single -> `Single_end | `Paired -> `Paired_end in
        let fastq_pair =
          let output_prefix = work_dir // to_file_prefix ?read:None what in
          let sample_name = Option.map info_opt (fun x -> x.sample_name) in
          Picard.bam_to_fastq ~run_with:machine ~sample_type
            ?sample_name
            ~output_prefix bam
        in
        fastq_pair
      | Paired_end_sample (info, l1, l2) ->
        let r1 = fastq_step ~compiler ~read:(`R1 info.fragment_id) l1 in
        let r2 = fastq_step ~compiler ~read:(`R2 info.fragment_id) l2 in
        let open KEDSL in
        workflow_node (fastq_reads ~host:Machine.(as_host machine)
                         ~name:info.sample_name
                         r1#product#path (Some r2#product#path))
          ~name:(sprintf "Pairing sample %s (%s and %s)"
                   info.sample_name
                   (Filename.basename r1#product#path)
                   (Filename.basename r2#product#path))
          ~edges:[ depends_on r1; depends_on r2 ]
      | Single_end_sample (info, single) ->
        let r1 = fastq_step ~compiler ~read:(`R1 info.fragment_id) single in
        let open KEDSL in
        workflow_node (fastq_reads ~host:Machine.(as_host machine)
                         ~name:info.sample_name
                         r1#product#path None)
          ~equivalence:`None
          ~edges:[ depends_on r1 ]
          ~name:(sprintf "single-end %s (%s)"
                   info.sample_name
                   (Filename.basename r1#product#path))
      | With_metadata (metadata_spec, p) ->
        fastq_sample_step ~compiler p |> apply_with_metadata ~metadata_spec

  and compile_aligner_step ~compiler (pipeline : bam pipeline) =
    let {reference_build; work_dir; machine; _} = compiler in
    let result_prefix = work_dir // to_file_prefix pipeline in
    dbg "Result_Prefix: %S" result_prefix;
    let rec parallelize_alignment ~make_aligner sample =
      match sample with
      | Paired_end_sample (info,
                           Gunzip_concat r1_list,
                           Gunzip_concat r2_list) ->
        let exploded =
          let count = ref 0 in
          List.map2 r1_list r2_list
            ~f:(fun r1 r2 ->
                match r1, r2 with
                | (Fastq_gz wf1, Fastq_gz wf2) ->
                  let new_info =
                    incr count; 
                    {info with
                     fragment_id =
                       (* fragmenting = creating fragments of previous fragment *)
                       sprintf "%s-fragment-%d" info.fragment_id !count} in
                  make_aligner (
                    Paired_end_sample (new_info,
                                       Gunzip_concat [r1],
                                       Gunzip_concat [r2]))
                | other -> failwith "compile_aligner_step: not implemented"
              ) in
        let bams = List.map exploded ~f:(compile_aligner_step ~compiler) in
        Samtools.merge_bams ~run_with:machine bams
          (result_prefix ^ "-merged.bam")
      | With_metadata (metadata_spec, p) ->
        parallelize_alignment ~make_aligner p
        |> apply_with_metadata ~metadata_spec
      | other ->
        failwith "parallelize_alignment: Not fully implemented"
    in
    let catch_parallelize_aligner aligner sample =
      let option =
        List.find_map compiler.options
          (function
          | `Parallel_alignment_over_fastq_fragments (al, todo)
            when List.mem ~set:al aligner -> Some todo
          | _ -> None)
      in
      match sample with
      | Paired_end_sample (name,
                           Gunzip_concat r1_list,
                           Gunzip_concat r2_list)
        when List.length r1_list > 1 ->
        begin match option with
        | Some _ -> `Caught
        | None -> `Not_caught
        end
      | Paired_end_sample (name,
                           Gunzip_concat r1_list,
                           Gunzip_concat r2_list)
        when List.length r1_list = 1 -> `Not_caught
      | other ->
        begin match option with
        | Some `Silent
        | None -> `Not_caught
        | Some `Fail_if_not_happening ->
          failwithf "Option Parallel_alignment_over_fastq_fragments is set to Fail_if_not_happening and it didn't happen:\n%s"
            (to_json other |> Yojson.Basic.pretty_to_string)
        end
    in
    let perform_aligner_parallelization aligner_tag ~make_aligner
        ~make_workflow sample =
      begin match catch_parallelize_aligner aligner_tag sample  with
      | `Caught -> parallelize_alignment ~make_aligner sample
      | `Not_caught ->
        let fastq = fastq_sample_step ~compiler sample in
        make_workflow fastq
      end
    in
    let bam_node =
      match pipeline with
      | Bam_sample (name, bam_target) -> bam_target
      | Gatk_indel_realigner (configuration, bam)
        when has_option compiler ((=) (`Map_reduce `Gatk_indel_realigner)) ->
        let input_bam = compile_aligner_step ~compiler bam in
        Gatk.indel_realigner_map_reduce ~run_with:machine ~compress:false
          ~configuration (KEDSL.Single_bam input_bam)
      | Gatk_indel_realigner (configuration, bam) ->
        let input_bam = compile_aligner_step ~compiler bam in
        Gatk.indel_realigner ~run_with:machine ~compress:false
          ~configuration (KEDSL.Single_bam input_bam)
      | Gatk_bqsr (configuration, bam) ->
        let input_bam = compile_aligner_step ~compiler bam in
        let output_bam = result_prefix ^ ".bam" in
        Gatk.base_quality_score_recalibrator
          ~configuration ~run_with:machine ~input_bam ~output_bam
      | Picard_mark_duplicates (settings, bam) ->
        let input_bam = compile_aligner_step ~compiler bam in
        let output_bam = result_prefix ^ ".bam" in
        Picard.mark_duplicates ~settings
          ~run_with:machine ~input_bam output_bam
      | Bwa_mem (bwa_mem_config, fq_sample) ->
        perform_aligner_parallelization
          `Bwa_mem ~make_aligner:(fun pe -> Bwa_mem (bwa_mem_config, pe))
          fq_sample
          ~make_workflow:(fun fastq ->
              Bwa.mem_align_to_sam ~reference_build
                ~configuration:bwa_mem_config
                ~fastq ~result_prefix ~run_with:machine ()
              |> Samtools.sam_to_bam ~reference_build ~run_with:machine)
      | Bwa (bwa_config, what) ->
        perform_aligner_parallelization
          `Bwa ~make_aligner:(fun pe -> Bwa (bwa_config, pe)) what
          ~make_workflow:(fun fastq ->
              Bwa.align_to_sam ~reference_build
                ~configuration:bwa_config
                ~fastq ~result_prefix ~run_with:machine ()
              |> Samtools.sam_to_bam ~reference_build ~run_with:machine)
      | Mosaik (what) ->
        perform_aligner_parallelization
          `Mosaik ~make_aligner:(fun pe -> Mosaik (pe)) what
          ~make_workflow:(fun fastq -> Mosaik.align ~reference_build
                ~fastq ~result_prefix ~run_with:machine ())
      | Star (configuration, what) ->
        perform_aligner_parallelization
          `Star ~make_aligner:(fun pe -> Star (configuration, pe)) what
          ~make_workflow:(fun fastq ->
              Star.align ~configuration ~reference_build
                ~fastq ~result_prefix ~run_with:machine ())
      | Hisat (configuration, what) ->
        perform_aligner_parallelization
          `Hisat ~make_aligner:(fun pe -> Hisat (configuration, pe)) what
          ~make_workflow:(fun fastq ->
              Hisat.align ~configuration ~reference_build
                ~fastq ~result_prefix ~run_with:machine ()
              |> Samtools.sam_to_bam ~reference_build ~run_with:machine
            )
      | With_metadata (metadata_spec, p)  ->
        compile_aligner_step ~compiler p
        |> apply_with_metadata ~metadata_spec
    in
    compiler.wrap_bam_node pipeline bam_node