[amibroker] lastvalue and N last quotations

[murthysuresh]

i add to watchlist in my scans using lastvalue(Buy)
however if i set my N last quotations >1 then it will not work properly as the 
Buy signal could have been in a earlier bar. how can i detect the value of the 
N last quotations and check for Buy signals within the N last quotations  and 
combine it with a if statement.
for eg this is my code which i need to convert to use the N last quotations.

if((LastValue(Buy)==1 )) {
CategoryAddSymbol("",categoryWatchlist,  wlnumber)
}