Manual example for building Emacs with packages does not build

Describe the problem

I was trying to use "Example 7. Nix expression to build Emacs with packages (emacs.nix)" from the NixOS manual as a example for my own configuration, and when it didn't work, I did a sanity check and tried building the example code. I copied and pasted it into a file named "emacs.nix" and ran nix-build emacs.nix as recommended by the manual.

Here are the errors that I got:

error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:37:12:
           36|
           37|   strict = derivationStrict drvAttrs;
             |            ^
           38|

       … while evaluating derivation 'emacs-with-packages-30.1'
         whose name attribute is located at /nix/store/nrfgfqpdc5rdx8dd4h7y60c3h0qi100y-nixos-25.05/nixos/pkgs/stdenv/generic/make-derivation.nix:480:13

       … while evaluating attribute 'deps' of derivation 'emacs-with-packages-30.1'
         at /nix/store/nrfgfqpdc5rdx8dd4h7y60c3h0qi100y-nixos-25.05/nixos/pkgs/applications/editors/emacs/build-support/wrapper.nix:63:5:
           62|     # one path to the load lists
           63|     deps =
             |     ^
           64|       runCommand "emacs-packages-deps"

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: undefined variable 'undo-tree'
       at /home/jjramsey/test-emacs-build/emacs.nix:40:5:
           39|   ++ (with epkgs.melpaPackages; [
           40|     undo-tree      # ; <C-x u> to show the undo tree
             |     ^
           41|     zoom-frm       # ; increase/decrease font size for all buffers %lt;C-x C-+>

Interestingly enough, this was similar to the error I got (except I was trying to install kdl-mode from melpaPackages), but got the same complaint about missing documentation

Proposed solution

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Start with Example 7, “Nix expression to build Emacs with packages (emacs.nix),” in the NixOS manual and reproduce the failure by copying it to emacs.nix and running nix-build emacs.nix. Check why the documented undo-tree package is undefined, then verify that the example builds successfully.