Why doesn't Info `T' cache node tree for current file?

[Drew Adams]

Info-toc seems to build the TOC from scratch (via Info-build-toc) each time it
is called. Just wondering why.

I understand that the TOC built depends on the current Info file, but I'm
wondering why the node list (tree) for a given Info file isn't kept around, to
avoid rebuilding it each time a user hits `T'.

Just trying to understand the code a bit better - not necessarily suggesting any
change is needed.