Art and sound effects: Kenney (www.kenney.nl), CC0 / public domain.
  - Space Shooter Redux        https://opengameart.org/content/space-shooter-redux
  - Puzzle Game Art            https://opengameart.org/content/puzzle-game-art
  - 63 Digital Sound Effects   https://opengameart.org/content/63-digital-sound-effects-lasers-phasers-space-etc
Pixel font: Press Start 2P by CodeMan38, SIL Open Font License 1.1.