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@mixin transition($duration:3s){

    -webkit-transition:$duration ease-out;

    -moz-transition:$duration ease-out;

    -ms-transition:$duration ease-out;

    -o-transition:$duration ease-out;

    transition:$duration ease-out;

}

 .move{

    @include transition(0.5s);

}

 .move-b{

    @include transition(); /* 引数渡してないのでデフォルト値の3sが使われる */

}