code: protected boolean isVisited(PRIndirectReference ref) {
          IntHashtable refs = (IntHashtable)visited.get(ref.getReader());
          if (refs != null)
          	return refs.containsKey(ref.getNumber());
          else
          	return false;
      }
docs: Checks if a reference has already been "visited" in the copy process.
  @param	ref	the reference that needs to be checked
  @return	true if the reference was already visited