You'll see here that the package emacs-minimal actually inherits from the emacs package definition, and then all fields afterwards are essentially overwrites of whatever was in the emacs package.
Unfortunately, I just noticed that the manual alludes to this, but actually never explicitly documents it in the "Defining Packages" section :/
http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/...
You'll see here that the package emacs-minimal actually inherits from the emacs package definition, and then all fields afterwards are essentially overwrites of whatever was in the emacs package.
Unfortunately, I just noticed that the manual alludes to this, but actually never explicitly documents it in the "Defining Packages" section :/