The ";" is a part of notation of commutators. For exapmle [D2, R U' R'] means execution: D2 R U' R' D2 R U R'. If theres R' (the setup move) and ";" it means that is starts with R' then you do commutator and when you finished you do R (undo the setup).

See this: https://www.speedsolving.com/wiki/index.php/Commutators_and_Conjugates#Short_notation

See this: https://www.speedsolving.com/wiki/index.php/Commutators_and_Conjugates#Short_notation

**real**math notation as well. But [A; B, C: D] is shorter than writing [A: [B, [C: D] ] ], and that's what I think Lucas was aimed at doing...getting rid of brackets.

So [A; B,C] is the proper way to write [A: [B,C] ] should you wish to use the semicolon. I will pm Lucas and ask him if this was his intent. If so, I (or I will get him) to state this in the wiki to minimize confusion. I prefer brackets myself over a semicolon, but if a lot of people like this abbreviated notation, I just want to set the necessary conditions to have it work for the community rather than against it. The semicolon should not be interchangeable with the colon!