/// Calc计算
/// @param {String} $prop CSS属性
/// @param {String} $val CSS属性值的表达式
/// @example
/// //SCSS
/// .calc{
/// 	 @include calc(width,'100% - 20px');
///	}
/// //CSS
/// .calc {
///		width: calc(100% - 20px);
///	}

@mixin calc($prop, $val) {
    #{$prop}: calc(#{$val});
}