ab: meta.feature_type == "ab"
                    if (params.fb_reference){
                        return [ meta, fastq ]
                    } else {
                        error ("Antibody reference was not specified. Please provide a reference file for feature barcoding (e.g. antibody measurements).\nPlease refer to https://www.10xgenomics.com/support/software/cell-ranger/latest/analysis/inputs/cr-feature-ref-csv for more details.")
                    }

ab: meta.feature_type == "ab"
                    if ((fastq.getName() == 'EMPTY') || params.fb_reference) { // Or something like that
                        return [ meta, fastq ]
                    } else {
                        error ("Antibody reference was not specified. Please provide a reference file for feature barcoding (e.g. antibody measurements).\nPlease refer to https://www.10xgenomics.com/support/software/cell-ranger/latest/analysis/inputs/cr-feature-ref-csv for more details.")
                    }