Genki is right for the use of the or block.
But to be honest, with the example in your and_or capx, it makes absolutely no sense.
Your parent event will only execute if the value of "a" is equal to 1.
Than you would like as a sub event to check if the value of "a" is 0.
As it is now, with no actions changing whatsoever the value of "a" is, it makes absolutely no sense.
I don't know where you're going with it.