This question evaluates the ability to design and implement an extensible prefix tree (trie), covering competencies in Unicode-aware string handling, memory and time optimization, concurrency or thread-safety considerations, and support for operations like insert, search, startsWith, countPrefix and erase.

Implement a prefix tree (trie) supporting insert(word), search(word), startsWith(prefix), countPrefix(prefix), and erase(word). Optimize for time and memory; handle Unicode characters; and ensure thread-safety or document assumptions. Provide Big-O complexity for each operation, outline test cases (including edge cases like duplicates and deletions of non-existent words), and discuss trade-offs versus alternative data structures.