How to step dependency in a step based on the output from 2 previous steps in CWL workflow

Question

[a.james]

Hello All,

I have a CWL workflow with 7 steps. And fourth step takes input from 2nd and 3rd step. How would I specify this dependency in CWL.

For example the workflow looks like following,

        #!/usr/bin/env cwl-runner
        cwlVersion: v1.0
        class: Workflow
        inputs:
    -------------
    ---------

outputs:
 alignment_out:
  type: File
  outputSource: star/star_bam
 expression_out:
  type: File
  outputSource: expressioncount/expression_out
 libsize:
  type: File
  outputSource: libsize_calculator/libsize
 Hypoxiaresult:
  type: Directory
  outputSource: hypoxia_calculator/Hypoxiaresult
 outFile:
   type: File[]
   outputSource: mdsum_generator/outFile
 results_move:
  type: Directory
  outputSource: results_mover/results_move

steps:
  star:
    run: star.cwl
    in:
     ........
    ...........
    out: [star_bam]
  expressioncount:
   run: count_expression.cwl
   in:
    ....................
    bam: star/star_bam
    exp_out: exp_out
   out: [expression_out]
  libsize_calculator:
   run: libsize_calculator.cwl
   in:
    samples: expressioncount/expression_out
    outputfile: outputfile
   out: [libsize]
  score_calculator:
   run:score.cwl
   in:
    **sample_input: **
    outputdir: outputdir
    ..............................
   out: [result]

Here in the above example *sample_input: * needs files from both libsize_calculator: and expressioncount: steps, in this case how can I specify it. Also the outputdir is directory, where I need the scores to written to.

The score.cwl looks like this:

cwlVersion: v1.0
class: CommandLineTool

requirements:
 InlineJavascriptRequirement: {}

baseCommand: [Rscript, scorefinder.R]

inputs:
 sample_input:
  type: File
  inputBinding:
   position: 1
 outputdir:
  type: string
  inputBinding:
   position: 2
 cancertype:
  type: string?
  inputBinding:
   position: 3
 sample_id:
  type: string?
  inputBinding:
   position: 4

outputs:
 Hypoxiaresult:
  type: Directory
  outputBinding:
   glob: $(runtime.outdir)

So this is what I tried:

   score_calculator:
       run: score.cwl
       in:
        sample_input: [expressioncount/expression_out, libsize_calculator/libsize]
        outputdir: outputdir

which is throwing error as:

Source
                                                                                                          'expression_out' of
                                                                                                          type "File" is
                                                                                                          incompatible
pipeline_part1.cwl:146:5:    with sink
                                                                                                            'sample_input' of
                                                                                                            type "File"
                                                                                                            source has
                                                                                                            linkMerge method
                                                                                                            merge_nested
pipeline_part1.cwl:142:10: Source 'libsize' of
                                                                                                          type "File" is
                                                                                                          incompatible
pipeline_part1.cwl:146:5:    with sink
                                                                                                            'sample_input' of
                                                                                                            type "File"
                                                                                                            source has
                                                                                                            linkMerge method
                                                                                                            merge_nested