sass set variable if not defined

$content: "First content";
$content: "Second content?" !default;
$new_content: "First time reference" !default;

#main {
  content: $content;
  new-content: $new_content;
}

/* 
 * Compiles to
 * #main {
 *   content: "First content";
 *   new-content: "First time reference"; }
 */