1. S Bradner and Harvard University. 1997. RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels Status. S Bradner and Harvard University. 1997. RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels Status.
2. G. Costagliola A. Delucia S. Orefice and G. Polese. 2002. A classification framework to support the design of visual languages. Journal of Visual Languages and Computing (2002). 10.1006/jvlc.2002.0234 G. Costagliola A. Delucia S. Orefice and G. Polese. 2002. A classification framework to support the design of visual languages. Journal of Visual Languages and Computing (2002). 10.1006/jvlc.2002.0234
3. Plex languages
4. M Fowler. 2018. UML Distilled: A Brief Guide to the Standard Object Modeling Language. Pearson Education. https://books.google.co.za/books?id=VTdtDwAAQBAJ M Fowler. 2018. UML Distilled: A Brief Guide to the Standard Object Modeling Language . Pearson Education. https://books.google.co.za/books?id=VTdtDwAAQBAJ