// center vertically and/or horizontally an absolute positioned element
  
  @mixin center($xy:xy) {
    @if $xy == xy {
      left: 50%;
      top: 50%;
      bottom: auto;
      right: auto;
      @include transform(translateX(-50%) translateY(-50%));
    }
    @else if $xy == x {
      left: 50%;
      right: auto;
      @include transform(translateX(-50%));
    }
    @else if $xy == y {
      top: 50%;
      bottom: auto;
      @include transform(translateY(-50%));
    }
  }