docs: def resources_gc_prefix(options, policy_config, policy_collection):
      """Garbage collect old custodian policies based on prefix.
  
      We attempt to introspect to find the event sources for a policy
      but without the old configuration this is implicit.
      """
  
      # Classify policies by region
      policy_regions = {}
      for p in policy_collection:
          if p.execution_mode == 'poll':
              continue
          policy_regions.setdefault(p.options.region, []).append(p)
  
      regions = get_gc_regions(options.regions)
      for r in regions:
          region_gc(options, r, policy_config, policy_regions.get(r, []))