code: def _get_callers(items, stage, special_cases=False):
      
      callers = utils.deepish_copy(_CALLERS[stage])
      if special_cases and "cnvkit" in callers:
          has_lumpy = any("lumpy" in get_svcallers(d) or "lumpy" in d["config"]["algorithm"].get("svcaller_orig", [])
                          for d in items)
          if has_lumpy and any("lumpy_usecnv" in dd.get_tools_on(d) for d in items):
              if stage != "initial":
                  del callers["cnvkit"]
          else:
              if stage != "standard":
                  del callers["cnvkit"]
      return callers
docs: Retrieve available callers for the provided stage.
  
      Handles special cases like CNVkit that can be in initial or standard
      depending on if fed into Lumpy analysis.