@Nullable
    @Contract(value = "null -> null; !null -> !null", pure = true)
    public static <T, U, R> BiFunction<T, U, R> wrapBiFunction(@Nullable BiFunction<T, U, R> fn) {
        if (fn == null) return null;
        else if (fn instanceof TtlEnhanced) return fn;
        else return new TtlBiFunction<>(fn);
    }
wrap {@link BiFunction} to TTL wrapper.

@param fn input {@link BiFunction}
@return Wrapped {@link BiFunction}
@see TtlUnwrap#unwrap(Object)
@since 2.12.4
ttl2-compatible/src/main/java/com/alibaba/ttl/TtlWrappers.java