code: function checkBlock(i) {
    return i < tokensLength && tokens[i].type === TokenType.LeftCurlyBracket ?
        tokens[i].right - i + 1 : 0;
  }
docs: Check if token is part of a block (e.g. `{...}`).
  @param {Number} i Token's index number
  @returns {Number} Length of the block