Solution
bam_for_ASE_ch
.groupTuple()
.phase(vcf_for_ASE)
.map{ left, right ->
def sampleId = left[0]
def bam = left[1]
def bai = left[2]
def vcf = right[1]
tuple(sampleId, vcf, bam, bai)
}
.set { grouped_vcf_bam_bai_ch }