code: @Nullable
    public String getXMLStringNotNull (@Nonnull final IPSElement aPSElement)
    {
      final String ret = getXMLString (aPSElement);
      if (ret == null)
        throw new IllegalStateException ("Failed to serialize the passed PS element: " + aPSElement);
      return ret;
    }
docs: Get the passed Schematron element as a String
  
  @param aPSElement
  The schematron element to convert to a string. May not be
  <code>null</code>.
  @return The passed element as a string and never <code>null</code>.
  @throws IllegalStateException
  if serialization failed