st: local variable and indexing into a variable

["NEYMOTIN, FLORENCE" <neymotin@calmail.berkeley.edu>]

Hi, I tried typing:

foreach var in x y {
replace `var'[_N]=0
}

then it considers [_N] a "weight" when clearly I want it to just change the last observation of 
both x and y to be zeroes.

alternatively, I tried the inner line as: ( replace `var[_N]'=0 )    and then it doesn't recognize 
var[_N] and thinks that it is an entirely new local reference.
Is there a correct syntax here, or does it look like there is some other sort of mistake in my 
coding ?
thanks,
Florence
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