Guix's G-Expressions: Embedding Lower-Level Code in Higher-Level Code

Guix uses Scheme for both high-level actions (like defining packages) and low-level actions (like building derivations). To embed lower-level code within higher-level code, it employs G-expressions. For example, in the `start` field of `wesnoth-shepherd-service`, `#~(...)` passes lower-level code, while `#$(...)` escapes higher-level code, which the compiler lowers to lower-level code. The `make-forkexec-constructor` function creates and executes child processes, offering features like setting user, group, umask, and environment variables.