code: public static synchronized Collection<Token> updateLocalTokens(Collection<Token> addTokens, Collection<Token> rmTokens)
      {
          Collection<Token> tokens = getSavedTokens();
          tokens.removeAll(rmTokens);
          tokens.addAll(addTokens);
          updateTokens(tokens);
          return tokens;
      }
docs: Convenience method to update the list of tokens in the local system keyspace.
  
  @param addTokens tokens to add
  @param rmTokens tokens to remove
  @return the collection of persisted tokens