text: function Calculator () {}
  
  Calculator.prototype._filter = function (values) {
    var len = values.length,
        filtered = values.filter(Number);
  
    if (len !== filtered.length) {
      throw new Error('All values must be numeric');
    }
  
    return values;
  };
  
  Calculator.prototype.add = function () {
    var values = this._filter([].slice.apply(arguments));
  
    if (values.length < 2) {
      throw new Error('Add expects a minimum of two arguments but ' + values.length + ' were given.')
    }
  
    return values.reduce(function (a, b) {
      return a + b;
    });
  };
  