_: self: _: let
  inherit (self) merge mkMerge;
in {
  # When the block has a `_type` attribute in the NixOS
  # module system, anything not immediately relevant is
  # silently ignored. We can make use of that by adding
  # a `__functor` attribute, which lets us call the set.
  merge = mkMerge [] // {
    __functor = self: next: self // {
      # Technically, `contents` is implementation defined
      # but nothing ever happens, so we can rely on this.
      contents = self.contents ++ [ next ];
    };
  };

  enabled  = merge { enable = true; };
  disabled = merge { enable = false; };
}