find(pattern, replacement) {
        pattern = regexify(pattern);

        let matches = [];
        this._sheets.forEach(sheet => {
            matches = matches.concat(sheet.find(pattern, replacement));
        });

        return matches;
    }
Find the given pattern in the workbook and optionally replace it.
@param {string|RegExp} pattern - The pattern to look for. Providing a string will result in a case-insensitive substring search. Use a RegExp for more sophisticated searches.
@param {string|function} [replacement] - The text to replace or a String.replace callback function. If pattern is a string, all occurrences of the pattern in each cell will be replaced.
@returns {boolean} A flag indicating if the pattern was found.