"cmd[nargs][default]{anything}": async ( tree , _ , { variables } ) => {
      tree = await ast.materialize(tree);
      const [ cmd , nargs , dflt , blob ] = await commandDefinitionParser["cmd[nargs][default]{anything}"](tree) ;
      return processCommandDefinition( variables.get('cmd') , cmd, [ nargs , dflt ,blob ]);
    } ,