code: def standardize_cnv_reference(data):
      
      out = tz.get_in(["config", "algorithm", "background", "cnv_reference"], data, {})
      cur_callers = set(data["config"]["algorithm"].get("svcaller")) & _CNV_REFERENCE
      if isinstance(out, six.string_types):
          if not len(cur_callers) == 1:
              raise ValueError("Multiple CNV callers and single background reference for %s: %s" %
                                  data["description"], list(cur_callers))
          else:
              out = {cur_callers.pop(): out}
      return out
docs: Standardize cnv_reference background to support multiple callers.