Multi-Layered Subscription & Discount Engine

Design a billing module for a SaaS platform. Users have different 'SubscriptionStates' (Trial, Active, PastDue, Canceled) which restrict features. Additionally, the system must apply multiple 'Discounts': 'First-Time User' (fixed $ off), 'Seasonal' (% off), and 'Referral' (stackable). Discounts have priority levels and some cannot be combined. The system must calculate the final price and determine which features are currently enabled for the user.

Key concepts

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