Write a histogram for repeated words in a tweet. Count words case-insensitively, treat contiguous ASCII letters and digits as a word, and return only words whose frequency is at least two. Implement def word_histogram(tweet):; return a dictionary mapping each normalized word to its count. Punctuation and whitespace are separators, and dictionary insertion order should follow each word's first occurrence.
[A-Za-z0-9]+def word_histogram(tweet):