[jdewerd]

Yes, excellent point! I originally omitted this detail for simplicity, but on reflection I don't think it actually achieved much in the way of simplifying the rule so I changed it to reflect reality. Thanks for pointing that out.

EDIT: the rush of people offering up sieve optimizations is pushing me back towards formulating the rule of thumb on a consecutive block of numbers, since it makes it very clear that these are not included, rather than implicitly or explicitly including some subset of them (implicit is bad because opacity, explicit is bad because complexity).