public function substract(Matrix $matrix)
{
if ($this->rows !== $matrix->getRows() or $this->columns !== $matrix->getColumns()) {
return false;
}
$result = clone $this;
for ($r = 0; $r < $this->rows; ++$r) {
for ($c = 0; $c < $this->columns; ++$c) {
$result->setElement($r, $c, $this->matrix[$r][$c] - $matrix->getElement($r, $c));
}
}
return $result;
}