code: @Nonnull
    public EChange removeFunction (@Nullable final XPathFunctionKey aKey)
    {
      if (aKey == null)
        return EChange.UNCHANGED;
      return m_aMap.removeObject (aKey);
    }
docs: Remove the function with the specified name.
  
  @param aKey
  The function key to be removed. May be <code>null</code>.
  @return {@link EChange}