code: function checkExtend1(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkExtendSelector(i)) i += l;
    else return 0;
  
    if (tokens[i + 1] &&
        tokens[i + 1].value === 'extend' &&
        (l = checkPseudoc(i))) i += l;
    else return 0;
  
    if (l = checkSC(i)) i += l;
  
    if (l = checkBlock(i)) i += l;
    else return 0;
  
    return i - start;
  }
docs: (1) `selector:extend(selector) {...}`