How can I partition a Lua file?

[Firesong]

Lua has a nice "require" system to allow long files to be partitioned. WoW disables this but I can see many addons have multiple files. What I can't work out is how they do it.

For example, if I have a behavoiur that is different for Tank and DPS, I'd love to be able to have the Tank stuff in a file called "Tank.lua"

How do I do this? Sorry if it's a really noob question.