Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens ?

Explanation

Sodium extract is boiled with nitric acid to decompose NaCN and `Na_(2)S`, If present, otherwise these
`NaCN + HNO_(3) rarr NaNO_(3) + HCN uarr`
`Na_(2)S + 2 HNO_(3) rarr 2 NaNO_(3) + H_(2)S uarr`
will react with `AgNO_(3)` and hence will interfere with the test as shown below:
`NaCN + AgNO_(3) rarr underset(underset("(White ppt.)")("Silver cyanide"))(AgCN)+ NaNO_(3)`
`Na_(2)S + 2 AgNO_(3) rarr underset(underset("(Black ppt.)")("Silver sulphide"))(Ag_(2)S)+ 2NaNO_(3)`