The Cellar of Sealed Casks

The inn at bitwick keeps a cellar with room for a fixed number of casks. Every cask has a name and a tier (an integer from 0 to 9, higher is finer), and every cask is either sealed or unsealed. When a cask arrives at a full cellar, something has to leave.

Build Cellar, constructed with capacity, an integer of 0 or more.

A cask is touched when it is successfully stored and when it is poured. Nothing else touches a cask. No two touches happen at the same moment, so "least recently touched" always names exactly one cask.

`store(name, tier)` — name is a string, tier an integer from 0 to 9. Returns a string or null:

• If a cask called name is already in the cellar: set its tier to tier, touch it, return null. Nothing is evicted and its seal is unchanged.
• Otherwise, if the cellar holds fewer than capacity casks: add it, unsealed, touch it, return null.
• Otherwise the cellar is full. If at least one cask in it is unsealed, remove one of them — the unsealed cask of lowest tier, and among those the least recently touched — then add the arriving cask, unsealed and touched, and return the removed cask's name. The arriving cask's own tier plays no part in choosing which cask is removed.
• Otherwise (full, with no unsealed cask in it) the arriving cask is not stored at all, the cellar is unchanged, and the return value is name itself.

`pour(name)` — if that cask is in the cellar, touch it and return its tier (an integer). If it is not, change nothing and return -1.

`seal(name)` — if that cask is in the cellar, mark it sealed and return true; sealing an already-sealed cask still returns true. If it is not there, change nothing and return false. Sealing does not touch a cask.

`unseal(name)` — the same, but marking the cask unsealed.

`contents()` — returns the names of the casks currently in the cellar, as a list of strings, ordered by tier descending; casks of equal tier are ordered most recently touched first.