code: function _parseParenthesizedSubformula(self, currentString) {
    if (currentString.charAt(0) !== '(') {
      return null;
    }
  
    const parsedSubformula = _parseFormula(self, sliceSymbol(currentString, '('), MIN_PRECEDENCE);
    if (parsedSubformula.remainder.charAt(0) !== ')') {
      throw new SyntaxError('Invalid formula! Found unmatched parenthesis.');
    }
  
    return {
      json: parsedSubformula.json,
      remainder: sliceSymbol(parsedSubformula.remainder, ')')
    };
  }
docs: Attempts to parse a parenthesized subformula at the head of a given string.
  Returns an AST node and string remainder if successful, otherwise null.
  
  @private
  @param {FormulaParser} self
  @param {string}        currentString - remainder of input string left to parse
  @returns {?Object}