Guix replaces the programming language in Nix with one written in Guile, a Scheme implementation that is the official extension language of the GNU project. Guix also contains package definitions for a fully free distribution of the GNU system. If an official, fully free GNU OS sounds awesome to you then you will like Guix.
Concrete paths inherit from pure paths - this would be quite strange using antonyms. This isn't like an "abstract" base class. It's a full class on its own, which is extended by another.
Anyway, I think it's meant to be "pure" like pure functions, generally having no side effects or external inputs.
i think so too. like it says in the docs: "Regardless of the system you’re running on, you can instantiate all of these classes, since they don’t provide any operation that does system calls." http://docs.python.org/3.4/library/pathlib.html#pure-paths
It's not a song, it's a prelude. And actually not very technically difficult, one reason for its immediate and enduring popularity. The simultaneous chords are just played very quickly one after the other and not "rolled".
"not very technically difficult" is a matter of perspective. It's about as difficult a piece as any student or amateur is ever going to play or be exposed to and few students or amateurs are skilled enough to play it.
But it is accessible to amateurs, and sounds very impressive. Which is, as you noted, a good reason why it's so popular. It's a good piece to pull out when you want to show off.
