{ # use -vW=4 or 14
	for (i=1; ; i++) {
		window = substr($0,i,W)
		for (j=1; j<=W; j++) {  # populate a Set
			A[substr(window,j,1)]=1
		}
		if (length(A) == W) {
			print i + W - 1
			next
		}
		delete A
	}
} # O(N*W*log2(W))