anchor.diff:  
       environment.etc."zinputrc".source = ./zinputrc;
   
  -    environment.systemPackages = [ pkgs.zsh ];
  +    environment.systemPackages = [ pkgs.zsh ]
  +      ++ optional cfg.enableCompletion pkgs.nix-zsh-completions;
   
       #users.defaultUserShell = mkDefault "/run/current-system/sw/bin/zsh";
   
  
positive.code:       enable = mkOption {
          default = false;
          description = ''
            Whenever to configure Zsh as an interactive shell.
          '';
          type = types.bool;
        };
  
negative.code:         type = types.lines;
        };
  
        enableCompletion = mkOption {
          default = true;
          description = ''
            Enable zsh completion for all interactive zsh shells.
          '';
          type = types.bool;
        };
  
      };
  
    };
  