Tree travel algorithm – misunderstanding

From Into To Algorithms. Here is the code:

I dont understand how it starts. y = null, in the first loop we will enter the else for post order – y = right[x] but thats not true. There is even no option for the first time that fit.

I putted an arrow where i think we will get in at the first time, and i dont understand why if we print there we will get postoder.

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