presto-main/src/main/java/io/prestosql/operator/scalar/WordStemFunction.java/WordStemFunction/wordStem
class WordStemFunction:    @Description("returns the stem of a word in the given language")
    @ScalarFunction
    @LiteralParameters("x")
    @SqlType("varchar(x)")
    public static Slice wordStem(@SqlType("varchar(x)") Slice slice, @SqlType("varchar(2)") Slice language)
    {
        Supplier<SnowballProgram> stemmer = STEMMERS.get(language);
        if (stemmer == null) {
            throw new PrestoException(INVALID_FUNCTION_ARGUMENT, "Unknown stemmer language: " + language.toStringUtf8());
        }
        return wordStem(slice, stemmer.get());
    }