/**
* $color 颜色
* $radius 弧度
* $positions: 从 'top-left', 'top-right', 'bottom-left', 'bottom-right'
* 选出任意N个有效值填入即可
**/
@mixin inner-round-corner ($color, $radius, $positions...)
// 调用example
@include inner-round-corner(red, 5px, 'top-left', 'top-right');