Lassaigne's test is used for the detection of which of the following?

What is the fundamental principle of the Lassaigne test?

Which of the following compounds will not give a positive Lassaigne's test for nitrogen?

The formula of the purple colour formed in $Lassaigne's$ test for sulphur using sodium nitroprusside is ......

Give the answers for the Lassaigne test:
$(i)$ If nitrogen and sulphur are both present,what observation is observed?
$(ii)$ If bromine is present,what is the observation?
$(iii)$ In the Lassaigne extract,if black precipitates are not obtained upon adding $CH_3COOH$ and lead acetate,what does this indicate?
$(iv)$ What is the reason for adding $FeSO_4$ to the Lassaigne extract?
$(v)$ Why is the Prussian blue colour observed?
$(vi)$ What does the violet colour with sodium nitroprusside in the Lassaigne extract indicate?

Why is the Lassaigne's extract boiled with concentrated $HNO_3$ before testing for halogens?