Software is complex. You wouldn’t believe how complex it is. You might think a skyscraper is a complex system, but if buildings got built like software, you would frequently run into fractal cathedrals filled with basketball halls, space ports and zero gravity hair styling salons, reaching into higher orbit and standing upside down on one of their many towers.
Because software is complex, people tend to reduce focus to what they think is most important and polish it as well as they can. However, every once in a while, one stumbles over something which can only be called a diamond because from whichever point of view you look at it, it always looks perfect. One such system is Mercurial, an SCM system.