Function operation

mini-code-editor/cm/lib/codemirror.js:1894–1901

(f)

    }
    var nestedOperation = 0;
    function operation(f) {
      return function() {
        if (!nestedOperation++) startOperation();
        try {var result = f.apply(this, arguments);}
        finally {if (!--nestedOperation) endOperation();}
        return result;
      };
    }

    function compoundChange(f) {
      history.startCompound();