Item compression in the Gist index

Hi,
we are developing an extension for multidimensional data. We have
created a Gist index that is heavily inspired by a cube extension. Right
now we would like to add some item compression since data items in a
node share a significant portion of a tuple prefix. However, I have no
idea how to handle information stored on a node level in Gists'
compression method. Is there any example? Unfortunately, the cube
extension does not implement item compression.
To be more specific we would like to store in the node a common prefix
for all tuples in the node.
Thanks for any advice,
Radim

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