<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _collapse_by_bam_variantcaller(samples):
 """Collapse regions to a single representative by BAM input, variant caller and batch. """

    by_bam = collections.OrderedDict()
    for data in (x[0] for x in samples):
        work_bam = utils.get_in(data, ("combine", "work_bam", "out"), data.get("align_bam"))
        variantcaller = get_variantcaller(data)
        if isinstance(work_bam, list):
            work_bam = tuple(work_bam)
        key = (multi.get_batch_for_key(data), work_bam, variantcaller)
        try:
            by_bam[key].append(data)
        except KeyError:
            by_bam[key] = [data]
    out = []
    for grouped_data in by_bam.values():
        cur = grouped_data[0]
        cur.pop("region", None)
        region_bams = cur.pop("region_bams", None)
        if region_bams and len(region_bams[0]) > 1:
            cur.pop("work_bam", None)
        out.append([cur])
    return out