How to use nested IF & ELSE condition in BEx Designer

While creating inventory ageing, how to use nested IF & ELSE conditions.

0 To 30 | 31 To 60 | 61 To 90 | 91 To 180 | >180

Logic..

0 to 30 Bucket

IF - Receipt Qty > Stock Qty

Then - Stock Qty.

ELSE - Receipt Qty.

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31 to 60 Bucket

IF - Receipt Qty > (Stock Qty - 0 To 30 Receipt Qty)

Then - Stock Qty - 0 To 30 Receipt Qty

(Here is the catch )

IF - (Stock Qty - 0 To 30 Receipt Qty) > 0

Then -  Result

Else - 0

Else - Receipt Qty of 31 To 60 Days

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I have understand what to do in 0 To 30 bucket, but I'm completely blank regarding 31 to 60 Days bucket. How we can take nested IF & ELSE