Norvig wrote a similar expansion in his chapter for the book Beautiful Data (along with several other small programs to do fun things with natural language corp

Norvig wrote a similar expansion in his chapter for the book Beautiful Data (along with several other small programs to do fun things with natural language corpus data). You can find it with a web search. (His version didn't use a trie, because Python's built-in dicts are much more efficient than a trie in Python, even with the extra redundancy that a trie eliminates.)