The YAZ Proxy is highly configurable and can be used in a number of different applications, ranging from debugging Z39.50-based applications and protecting overworked servers, to improving the performance of stateless WWW/Z39.50 gateways. (taken from https://www.indexdata.com/resources/software/yazproxy/) YAZProxy depends on the YAZ++-API (dev-libs/yazpp), which depend on the YAZ Toolik (dev-libs/yaz)
see also https://github.com/gentoo/gentoo/pull/9635