public function sub($subtrahend)
{
if ($subtrahend instanceof NumArray) {
$subtrahend = $subtrahend->getData();
}
$this->data = Map::mapArray($this->data, $subtrahend, function ($data1, $data2) {
return $data1 - $data2;
});
$this->shape = Shape::getShape($this->data);
$this->flushCache();
return $this;
}