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How to Set Boolean attribute to null

I am implement nested Boolean condition (yes/No), and I don't want default value to set either true or false

so Let say attribute

when A changes to = No ; only at that time I take Boolean value for attribute for B.

My question is how to set Boolean attribute to null or clear it.

Because in my case the processes in loop, even I don't set value for attribute B, is still sets to false.

I must clear this if A attribute is yes