src/Collection.php
private function appendToArray(string $key, $value): self
    {
        $this->props[$key] = [$this->props[$key], $value];

        return $this;
    }
private function appendToArray(string $key, $value): self
    {
        $this->props[$key] = [$this->props[$key], $value];

        return $this;
    }
appending to already existing array

@param mixed $value