I don't think it's a matter of being cost-effective. It is simply that there's basically a list, and I think the other one, the dinitrate, is an older drug and was put on the list at some point. When the new one came in, there was no demand for zero-rating that product immediately. It came about because we have been made aware of that change.
On May 16th, 2012. See this statement in context.