presto-main/src/main/java/io/prestosql/metadata/FunctionExtractor.java/FunctionExtractor/extractFunctions
class FunctionExtractor:    public static List<? extends SqlFunction> extractFunctions(Collection<Class<?>> classes)
    {
        return classes.stream()
                .map(FunctionExtractor::extractFunctions)
                .flatMap(Collection::stream)
                .collect(toImmutableList());
    }