An overlay could define a package with the same name as one from a lower level. IE python = super.python... It could also completely override the entire defin

An overlay could define a package with the same name as one from a lower level. IE python = super.python... It could also completely override the entire definition of something if you wanted. IE python = super.stdenv.mkDerivation { ... }; But you can also add new names, and not modify the existing definitions. IE python-for-me = super.python.overrideAttrs (...); In the last case you can reference your modified copy while other things in nixpkgs can use the one built upstream.