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In March/April 2013, the major CS1 cite templates were transitioned to use the Lua programming language. Each of the updated templates should render citations just like the old template in most cases. The Lua-based templates are more efficient and use fewer resources than the prior markup-based templates which used Template:Citation/core, and thus pages will load faster during edit-preview. If you see a problem, please report it on the talk page, WT:CS1.
Another improvement is in the number of authors and editors: these are now unlimited. For example, you can use last1 through last100 and beyond, using common sense of course. There is also more error checking: we haven't added more red error messages yet, but we do place pages in error categories.
There have been some outstanding issues that have been resolved that were difficult to fix or where the fix caused performance issues in the old template.
The documentation will be updated soon to reflect these changes.