//扩展Extend

Use Method：以在study上扩展指定多层嵌套选择器样式

//Share style 
.test{
    font-size:18px;
    color:#ffffff;
    ul{
        width:100px; 
        height:50px;
        li{
            float:left;
            margin-right:10px;
            width:50px;
            height:50px;
        }
    }
}

//grammar 1
.study{
    &:extend(.test ul li);
}

//grammar 2
.study:extend(.test ul li){}

//输出css
.test {
  font-size: 18px;
  color: #ffffff;
}
.test ul {
  width: 100px;
  height: 50px;
}
.test ul li,
.study {
  float: left;
  margin-right: 10px;
  width: 50px;
  height: 50px;
}