public function append(string $key, $val) : self
{
if (!isset($this->stored[$key])) {
$this->stored[$key] = [];
} elseif (!is_array($this->stored[$key])) {
// Let's store the existing value as the first index
$this->stored[$key] = [$this->stored[$key]];
}
$this->stored[$key][] = $val;
return $this;
}