@Nullable
    @Contract(value = "null -> null; !null -> !null", pure = true)
    public static <T> Callable<T> unwrap(@Nullable Callable<T> callable) {
        if (!(callable instanceof TtlCallable)) return callable;
        else return ((TtlCallable<T>) callable).getCallable();
    }
Unwrap {@link TtlCallable} to the original/underneath one.
<p>
this method is {@code null}-safe, when input {@code Callable} parameter is {@code null}, return {@code null};
if input {@code Callable} parameter is not a {@link TtlCallable} just return input {@code Callable}.
<p>
so {@code TtlCallable.unwrap(TtlCallable.get(callable))} will always return the same input {@code callable} object.

@see #get(Callable)
@see TtlWrappers#unwrap(Object)
ttl-core/src/main/java/com/alibaba/ttl3/TtlCallable.java