Notation question

I'm describing a model fitting procedure and using 'x' as a shorthand to denote a list of all the free model parameters. I want to clearly state the contents of x in a formal way rather than just list the contents of x in a sentence. Is there a standard notation for doing this?

For instance suppose x contains variables A, B, C, and D. I was thinking something like this might do it...

http://imgur.com/WoKhs2G

But I'm not sure if that's the conventional way to do it. For instance the "equals" sign just seems a little off in this usage. Any input?