code: function _parseVariable(self, currentString) {
    const variable = (currentString.match(/^\w+/) || [])[0];
    if (!variable) {
      return null;
    }
  
    return {
      json: { [self.variableKey]: variable },
      remainder: sliceSymbol(currentString, variable)
    };
  }
docs: Attempts to parse a variable (i.e., any alphanumeric substring) 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}