The difference is that people can export from outside a CustomSourceLanguage with a string representing the language, while the UnknownSourceLanguage is provided by default (null object pattern). So, the CustomSourceLanguage is simply a possibility to still specify some strange language from an outside parser without requiring a change in the Smalltalk code. For example, when you implemented the C# parser, you could export the language information even if we did not modify the Smalltalk code. Doru On Wed, Nov 23, 2011 at 3:15 PM, Usman Bhatti <usman.bhatti(a)gmail.com> wrote: -- www.tudorgirba.com "Every thing has its own flow"