What is a Compressed Prefix B+-Tree (CPBTree) in SAP HANA?

I'm studying the SAP HANA main memory database. There is an index called CPBTree in it. In its documentation, it is described as follows: > CPB+-tree stands for Compressed Prefix B+-Tree; this index tree type > is based on pkB-tree. CPB+-tree is a very small index because it uses > 'partial key' that is only part of full key in index nodes. This is a bit vague. There are no other explanations about the CPBTree's structure on the Internet. Is there anyone who can provide an explanation more or refer me to good documentation/URLs/&c.?