interface.ts

interface Account {
  subscriptions: AccountSubscriptions
  // ...
}

interface AccountSubscriptions {
  customer: AccountDID

  request(): Subscription
  list(): Subscription[]

  // Verifies that plan.customer === this.customer
  add(plan: Plan): Subscription
  remove(subscription: Subscription): {}
}

type Subscription = Variant<{
  active:       ActiveSubscription
  pending:      PendingSubscription   
}>
  
interface PendingSubscription {
   // URL user can navigate to complete subscription setup
   url: URL
   customer: AccountDID
}
  
interface ActiveSubscription extends UCAN<{ with: ServiceDID: can: "subscription/*", nb: { customer: AccountDID } }> {
   plan: Plan // terms of service
   customer: AccountDID  // Account that has a subscription
  
   provisions: SubscriptionProvisions
  
   // ensures that plan.customer === this.customer
   update({ plan: Plan }): void
}
  
 interface Plan extends UCAN<{ with: ProviderDID, nb: { customer: AccountDID } }> {
    price: Price
    provider: ProviderDID
    customer: AccountDID
    protocol: Record<Ability, Schema>
 }
  
 interface Price extends Record<string, number> {}

  
interface SubscriptionProvisions {
  list(): Provision[]
  add(provision: Provision): {}
  remove(provision: Provision): {}
}

  
interface Provision {
  consumer: SpaceDID
  limits: Limits
}
 
interface Limits extends Record<PropertyKey, never> {
}