code: function checkAtruler(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkAtkeyword(i)) i += l;
    else return 0;
  
    if (l = checkTsets(i)) i += l;
  
    if (l = checkAtrulers(i)) i += l;
    else return 0;
  
    return i - start;
  }
docs: Checks if token is part of an @-rule with ruleset.
  
  @param {number} i Token's index number
  @return {number} Length of the @-rule