Build a simple in-memory database that processes a sequence of timestamped commands and returns results for each command. Input: an array of commands, where each command is an array of strings. Output: an array of strings with the result for each command (use empty string "" for commands that return nothing). Timestamps are unique and strictly increasing. Keys and fields are strings; values are integers. The system evolves through four feature levels; all features coexist and must be supported together: Core commands (Level 1) - SET Add or update field=value in record key. Create the record if missing. Returns "". - COMPARE_AND_SET If field exists and its current (non-expired) value equals expectedValue, update it to newValue and return "true"; otherwise do nothing and return "false". - COMPARE_AND_DELETE If field exists and its current (non-expired) value equals expectedValue, delete the field and return "true"; otherwise return "false". - GET Return the current value as a string if the field exists and is not expired; otherwise return "". Filtering (Level 2) - SCAN Return a single string listing all non-expired fields in key in lexicographic order as: "f1(v1), f2(v2), ...". If no fields, return "". - SCAN_BY_PREFIX Like SCAN but only include fields whose names start with prefix. TTL / Expiration (Level 3) - SET_WITH_TTL Add or update field with value and an expiration at (timestamp + ttl). The value is valid for times t where timestamp <= t < timestamp + ttl. If updated, the TTL is overwritten. All other commands treat expired fields as absent at the current command timestamp. History (Level 4) - GET_WHEN Return the value the field had at time at_timestamp (as a string), considering the TTLs and changes that were effective at that past moment. You are guaranteed at_timestamp <= timestamp. If at_timestamp == 0, treat as a regular GET at the current timestamp. If nothing existed then, return "". Notes and semantics summary - Commands are processed in chronological order given by their timestamp. - A field that expires at time T is considered dead and inaccessible at timestamp T and later (unless re-set before or at T with a new TTL/value). - When a field is updated, its new value and TTL replace previous ones (but history must preserve prior states for GET_WHEN queries). Example (Level 1): Input: [ ["SET","0","A","B","4"], ["GET","1","A","B"], ["COMPARE_AND_SET","2","A","B","4","9"], ["GET","3","A","B"] ] Output: ["","4","true","9"]