anchor.diff:  
       home.sessionVariables =
         mapAttrs (n: v: toString v) (
  -        filterAttrs (n: v: v != []) {
  +        filterAttrs (n: v: v != [] && v != null) {
  +          FZF_ALT_C_COMMAND = cfg.changeDirWidgetCommand;
             FZF_ALT_C_OPTS = cfg.changeDirWidgetOptions;
  +          FZF_CTRL_R_COMMAND = cfg.historyWidgetCommand;
             FZF_CTRL_R_OPTS = cfg.historyWidgetOptions;
  +          FZF_CTRL_T_COMMAND = cfg.fileWidgetCommand;
             FZF_CTRL_T_OPTS = cfg.fileWidgetOptions;
  +          FZF_DEFAULT_COMMAND = cfg.defaultCommand;
             FZF_DEFAULT_OPTS = cfg.defaultOptions;
           }
         );
  
positive.code:       '';
      };
  
      changeDirWidgetOptions = mkOption {
        type = types.listOf types.str;
        default = [];
        example = [ "--preview 'tree -C {} | head -200'" ];
        description = ''
          Command line options for the ALT-C keybinding.
        '';
      };
  
  
negative.code:   options.programs.fzf = {
      enable = mkEnableOption "fzf - a command-line fuzzy finder";
  
      defaultCommand = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "fd --type f";
        description = ''
          The command that gets executed as the default source for fzf
          when running.
        '';
      };
  
      defaultOptions = mkOption {
        type = types.listOf types.str;
        default = [];
        example = [ "--height 40%" "--border" ];
        description = ''
          Extra command line options given to fzf by default.
        '';
      };
  
      fileWidgetCommand = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "fd --type f";
        description = ''
          The command that gets executed as the source for fzf for the
          CTRL-T keybinding.
        '';
      };
  
  