docs: function(x, x0, delta) {
           var absMax;
  
           delta = delta || utils.precision;
           absMax = Math.max(Math.abs(x0), Math.abs(x));
           if (isNaN(absMax)) { return isNaN(x) && isNaN(x0); } /* both NaN */
           if (absMax === Infinity) { return x0 === x; }
           if (absMax === 0) { return true; }
           if (x === 0) { return utils.isEssentiallyZero(x0); }
           if (x0 === 0) { return utils.isEssentiallyZero(x); }
           return Math.abs(x - x0) / absMax < delta;
        }