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Porter (1980) stemming algorithm.
Reference: M. F. Porter, “An Algorithm for Suffix Stripping”, Program
14(3), 1980. Note that this is the original 1980 algorithm, not Porter2
(Snowball English) — they differ on edge cases such as agreed and
feedeing. The output is intentionally identical to the upstream
UQA stemmer contract so that BM25 doc frequencies match across
engines.