Lua is a programming language that has existed for 15 years. I had heard of it many years ago, but unfortunately, it had slipped my mind. I became interested in Lua when I discovered what the PowerDNS Recursor does with it. In fact, I became so interested, that literally two weeks before publication of my book, I was able to squeeze in an appendix with a short overview of Lua, and what it does together with the Recursor. Lua is an embeddable language: you stick it into your C program, which then calls Lua functions you write. The other way is also possible: Lua functions that call your own "host" C functions. This is all very impressive, and worth a good look if you like programming. One of Lua's strengths is its excellent documentation; both the reference manual and the book Programming in Lua are online. The Lua Wiki is excellent, and it contains good pointers for starting as well as examples for advanced users.