code: boolean addContext(boolean isRoot, Vector names, Mode mode) {
      if (modeMap == null)
        modeMap = new ContextMap();
      return modeMap.put(isRoot, names, mode);
    }
docs: Adds a new context (isRoot, path --> mode).
  @param isRoot Flag indicating that the path starts or not with /
  @param names The local names that form the path.
  @param mode The mode for this path.
  @return true if we do not have a duplicate path.