code: private function getIndentAt(Tokens $tokens, $index)
      {
          if (1 === Preg::match('/\R{1}([ \t]*)$/', $this->getIndentContentAt($tokens, $index), $matches)) {
              return $matches[1];
          }
  
          return null;
      }
docs: @param Tokens $tokens
  @param int    $index  index of the indentation token
  
  @return null|string