struct Details { let cachedTokens: UInt64 }
struct Usage { let details: Details{{ optional }}; let totalTokens: UInt64 }
struct Chunk { let usage: Usage? }
let chunks = [Chunk(usage: Usage(details: Details(cachedTokens: 18), totalTokens: 18))]
precondition({{ expression }} == 18)
precondition({{ standard }} == 18)
let absent = [Chunk(usage: nil)]
precondition({{ absent }} == nil)