nitpick, but I think "existence proof" is the opposite of what you meant (that Mercurial is a living proof that things can be better):

> a constructive proof is a method of proof that demonstrates the existence of a mathematical object by creating [...] the object.

> This is in contrast to [an existence proof], which proves the existence of a particular kind of object without providing an example.

https://en.m.wikipedia.org/wiki/Constructive_proof