upto(n) = {twins = list(n); res = List(); 
twinchar = vector(n); 
for(i = 1, #twins, 
	for(j = i, #twins, 
		c = twins[i] + twins[j]; 
		if(c > n,
			next(2)
		,
			twinchar[c]++
		)

		
	)
); print(twinchar);
forprime(p = 11, n, 
	if(twinchar[p-3] == 0,
		listput(res, p);
	)
); res
}

\\ Function below by Charles R. Greathouse IV at A001097
list(lim)=my(v=List([3]), p=5); forprime(q=7, lim, if(q-p==2, listput(v, p); listput(v, q)); p=q); if(p+2>lim && isprime(p+2), listput(v, p)); Vec(v)