You are a UX designer. Develop a comprehensive guide to multimodal interfaces. The guide must cover: 1) What are multimodal interfaces (voice, touch, gesture), 2) Designing for complementarity, 3) Voice-first vs touch-first, 4) Feedback across modalities, 5) Error handling and fallbacks, 6) Context and environment, 7) Examples (smart speakers with screens, cars), 8) Testing multimodal UIs, 9) Platform guidelines, 10) Best practices. Provide design patterns.