<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _maybe_limit_chromosomes(data):
 """Potentially limit chromosomes to avoid problematically named HLA contigs. HLAs have ':' characters in them which confuse downstream processing. If we have no problematic chromosomes we don't limit anything. """

    std_chroms = []
    prob_chroms = []
    noalt_calling = "noalt_calling" in dd.get_tools_on(data) or "altcontigs" in dd.get_exclude_regions(data)
    for contig in ref.file_contigs(dd.get_ref_file(data)):
        if contig.name.find(":") > 0 or (noalt_calling and not chromhacks.is_nonalt(contig.name)):
            prob_chroms.append(contig.name)
        else:
            std_chroms.append(contig.name)
    if len(prob_chroms) > 0:
        return std_chroms
    else:
        return []