Merge grouped files from a output channel

[zillurbmb51]

Hi,
I was trying to merge chunks of chromosomes into respective chromosome. I tried many ways for many days, posted the issue in github (#2138), stackoverflow (https://stackoverflow.com/questions/67591229/output-files-chromosomal-chunks-merging-in-nextflow) and gitter but still failed to accomplish the task.

The chunks are stored in tuples of output channel, like:
[chr1,[[chr1.chunk1.bgen],[chr1.chunk1.stats]], [chr1,[[chr1.chunk2.bgen],[chr1.chunk2.stats]],[chr1,[[chr1.chunk3.bgen],[chr1.chunk3.stats]]...
How could I get chr1.merged.bgen and chr1.merged.stats ? I may use cat or other tools.
I tried to gather all the bgen files together:
my_output_channel.map{chrom,bgens -> tuple(chrom,bgens.findAll{it.endsWith(".bgen")})}.view()
But It gives me empty output [chr1,[]], [chr1,[]],[chr1,[]]...
Any help?