# my_xmodmap.nix
''
  clear mod1
  clear mod4
  keycode 108 = Super_R NoSymbol Super_R
  keycode 134 = Alt_R Meta_R Alt_R Meta_R
  add mod1 = Alt_L Alt_R
  add mod4 = Super_L Super_R
''

# my_config.nix
environment.etc."Xmodmap".text = import ./my_xmodmap.nix;