How to merge multiple grouped output files

[zillurbmb51]

Hi, I have a process that outputs multiple files as tuple. Like this:
[chr1,[[chr1.chunk1.bgen],[chr1.chunk2.bgen],[chr1.chunk3.bgen],[chr1.chunk1.stat],[chr1.chunk2.stat],[chr1.chunk3.stat]]]

The process that gave me those chunks is:

process imputation {
publishDir params.out, mode:'copy'
input:
tuple val(chrom),val(chunk_array),val(chunk_start),val(chunk_end),path(in_haps),path(refs),path(maps) from imp_ch
output:
tuple val("${chrom}"),path("${chrom}.*") into imputed
script:
def (haps,sample)=in_haps
def (haplotype, legend, samples)=refs
"""
impute4.1.2_r300.3 -g "${haps}" -h "${haplotype}" -l "${legend}" -m "${maps}" -o "${chrom}.step10.imputed.chunk${chunk_array}" -no_maf_align -o_gz -int "${chunk_start}" "${chunk_end}" -Ne 20000 -buffer 1000 -seed 54321

if [[ \$(gunzip -c "${chrom}.step10.imputed.chunk${chunk_array}.gen.gz" | head -c1 | wc -c) == "0" ]]
then 
 echo  "${chrom}.step10.imputed.chunk${chunk_array}.gen.gz" is empty
else
 qctool_v2.0.8_rhel -g "${chrom}.step10.imputed.chunk${chunk_array}.gen.gz" -snp-stats -osnp "${chrom}.step10.imputed.chunk${chunk_array}.snp.stats"
 qctool_v2.0.8_rhel -g "${chrom}.step10.imputed.chunk${chunk_array}.gen.gz" -og "${chrom}.step10.imputed.chunk${chunk_array}.bgen" -os "${chrom}.step10.imputed.chunk${chunk_array}.sample"
fi
 """
}

How could I get chr1.merged.bgen and chr1.merged.stat . I want to use cat to merge all these chunks.

I tried:

input:
tuple val (chrom), file('*.bgen'),file('*.stat') from imputed
"""
cat "${chrom}.${*.bgen}" > "${chrom}.merged.bgen"
cat "${chrom}.${*.stat}" > "${chrom}.merged.stat"
"""

But got " Input tuple does not match input set cardinality decalred
I also tried to use imputed.collect() and imputed.toList() But got same error.
Any help?