code: @Nonnull
    public EChange addUniqueFunction (@Nonnull final String sNamespaceURI,
                                      @Nonnull final String sLocalPart,
                                      @Nonnegative final int nArity,
                                      @Nonnull final XPathFunction aFunction)
    {
      return addUniqueFunction (new QName (sNamespaceURI, sLocalPart), nArity, aFunction);
    }
docs: Add a new function.
  
  @param sNamespaceURI
  The namespace URI of the function
  @param sLocalPart
  The local part of the function
  @param nArity
  The number of parameters of the function
  @param aFunction
  The function to be used. May not be <code>null</code>.
  @return {@link EChange}