Furthermore, since the original Buckwalter scheme was developed, several other variants have emerged, although they are not all standardized. Buckwalter transliteration is not compatible with XML, so "XML safe" versions often modify the following characters: < > & (أ إ and ؤ respectively; Buckwalter suggests transliterating them as I O W, respectively). Completely "safe" transliteration schemes replace all non-alphanumeric characters (such as $';*) with alphanumeric characters. For a complete description of different Buckwalter schemes as well as a more detailed discussion of the trade-offs between different schemes, see.