code: function checkAtrule(i) {
    let l;
  
    if (i >= tokensLength) return 0;
  
    // If token already has a record of being part of an @-rule,
    // return the @-rule's length:
    if (tokens[i].atrule_l !== undefined) return tokens[i].atrule_l;
  
    // If token is part of an @-rule, save the rule's type to token.
    if (l = checkKeyframesRule(i)) tokens[i].atrule_type = 4;
    // @-rule with ruleset:
    else if (l = checkAtruler(i)) tokens[i].atrule_type = 1;
    // Block @-rule:
    else if (l = checkAtruleb(i)) tokens[i].atrule_type = 2;
    // Single-line @-rule:
    else if (l = checkAtrules(i)) tokens[i].atrule_type = 3;
    else return 0;
  
    // If token is part of an @-rule, save the rule's length to token:
    tokens[i].atrule_l = l;
  
    return l;
  }
docs: Check if token is a part of an @-rule
  @param {Number} i Token's index number
  @returns {Number} Length of @-rule