-<!--
-we depend on an external entity here, which we cannot control. Suppose
-we go from 2.15.x to 2.17.x and put the documentation under "v2.17".
-For some time, Google won't have the new location in its index, so the
-search would get nothing. It would be better to keep "v2.15" in the
-search for a while and have a redirection from "v2.15" to "v2.17".
--->