In SGML, XML, and HTML, the ampersand is used to introduce an SGML entity, such as (for non-breaking space) or (for the Greek letter α). The HTML and XML encoding for the ampersand character is the entity . This can create a problem known as delimiter collision when converting text into one of these markup languages. For instance, when putting URLs or other material containing ampersands into XML format files such as RSS files the & must be replaced with &amp; or they are considered not well formed, and computers will be unable to read the files correctly. SGML derived the use from IBM Generalized Markup Language, which was one of many IBM-mainframe languages to use the ampersand to signal a text substitution, eventually going back to System/360 macro assembly language.