Simple World Language, or SWL for short, is the language in which the code of the bugs is written. SWL is a assembler language because, for several reasons, it needs to be a low level language:

1. Understanding of the mutations: each mutation can be seen as a instruction in SWL.
2. Dispersion of the code: small mutations become in small changes in the actions of the bug.

You can found fibonacci.swl attached, a example of working SWL code.

You can found a full description of the language in the specification.