static evalDivision(exprString) {
    let res;
    let n;
    if ((res = exprString.match(/^(\d+)\/(\d+)$/))) {
      n = Number(res[1]) / Number(res[2]);
    } else {
      n = Number(exprString);
    }
    return !isNaN(n) ? n : null;
  }
Evaluate a division expressed as a string.
@param {string} exprString
  The expression to evaluate, e.g. "200/2". Can also be a single number.
@return {?number} The evaluated expression as floating point number on
  success; otherwise return null.