Immutables

• New key_at() reads the key of an ordered_sequence element at a given one-based position without removing it (the positional companion to peek_at(), and the general form of min_key() / max_key()).
• New nearest_key() returns the existing key closest to a query key, composing with peek_key() / pop_key(). It resolves by order alone at the extremes and on an exact hit (any key type), and uses abs(query - key) when the query falls strictly between two distinct keys (numeric/Date/POSIXct). The ties argument resolves an equidistant tie between two distinct keys: "lower" (default), "upper", or "both" (returns both keys). It errors for non-subtractable keys (e.g. character) in that between case, where "closer" is undefined.

Performance

• ordered_sequence key-boundary lookups now use a native C++ descent for numeric, character, and logical keys, speeding up lower_bound(), upper_bound(), peek_key(), pop_key(), peek_all_key(), pop_all_key(), count_key(), elements_between(), and count_between().
• The implementation now uses lazy evaluation matching the Hinze and Paterson reference implementation, making amortized-constant-time end access claims true when structures are used in a persistent setting.

Documentation and fixes

• Minor documentation improvements
• Trimming of unit tests to speed check times on CRAN machines