//@include a(white, blue, green, red);
@mixin a($link, $visit, $hover, $active) {
  a {
    color: $link;
    &:visited {
      color: $visit;
    }
    &:hover {
      color: $hover; 
      text-decoration: none;
    }
    &:active {
      color: $active;
    }
  }
}