anchor.diff:  
   let
   
  -  inherit (lib) mkIf mkOption mkEnableOption types filterAttrs
  -                mapAttrsToList concatStringsSep;
  -  inherit (types) int bool nullOr attrsOf str either enum submodule lines path;
  +  inherit (builtins)
  +    filter isBool toString ;
   
  -  inherit (builtins) isBool filter toString;
  +  inherit (lib)
  +    concatMapStrings concatStringsSep filterAttrs mapAttrsToList mkEnableOption
  +    mkIf mkOption ;
  +
  +  inherit (lib.types)
  +    attrsOf bool either enum int lines nullOr path str submodule ;
  +
   
     cfg = config.nixsap.apps.nginx;
     explicit = filterAttrs (n: v: n != "_module" && v != null);
  
positive.code:       fastcgi_param HTTP_PROXY "";
        proxy_set_header Proxy "";
  
        ${concatStringsSep "\n" (mapAttrsToList mkServer cfg.http.servers)}
      }
    '';
  
  
negative.code:     let mkEntry = k: v: "${indent}${k} ${show v};";
      in concatStringsSep "\n" (mapAttrsToList mkEntry (explicit set));
  
    mkServer = name: text: pkgs.writeText "nginx-${name}.conf" ''
      # ${name}:
      server {
      ${text}
      }
  