The LSL compiler is generated with "Compiler tools in C#" version 4.7[1] by Dr. Malcolm Crowe[2]. The code is used with permission by the author:

There is no problem with using the code in any way you like, as long as
somewhere you say that that is what you have done (in the source for
example).
And of course we disclaim all responsibility for any resulting
damage...
Best wishes
Malcolm Crowe

[1] http://cis.paisley.ac.uk/crow-ci0/CSTools47.zip
[2] http://cis.paisley.ac.uk/crow-ci0/

Patch from Mike: clarification of Tools.dll licensing (used by the LLScript compiler)
Patch from Mike: clarification of Tools.dll licensing (used by the LLScript compiler)

