$a(); // read
$a.set(20); // write (1)
$a.update((prev) => prev + 10); // write
why not $a(20) and $a(prev => prev + 10)? $a(); // read
$a.set(20); // write (1)
$a.update((prev) => prev + 10); // write
why not $a(20) and $a(prev => prev + 10)?