code: function checkParentheses(i) {
    if (i >= tokensLength) return 0;
  
    const start = i;
    const right = tokens[i].right;
    let l;
  
    // Skip `(`.
    if (tokens[i].type === TokenType.LeftParenthesis) i++;
    else return 0;
  
    if (i < right) {
      if (l = checkTsets(i)) i += l;
      else return 0;
    }
  
    // Skip `)`.
    i++;
  
    return i - start;
  }
docs: Check if token is part of text inside parentheses, e.g. `(1)`
  @param {number} i Token's index number
  @return {number}