code: function _parseUnarySubformula(self, currentString) {
    const unary = matchOperator(currentString, self.unaries);
    if (!unary) {
      return null;
    }
  
    const parsedSubformula = _parseFormula(self, sliceSymbol(currentString, unary.symbol), unary.precedence);
  
    return {
      json: { [unary.key]: parsedSubformula.json },
      remainder: parsedSubformula.remainder
    };
  }
docs: Attempts to parse a unary 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}