help with rb-tree/update!

[JP de Vooght]

Hello,

I was looking into rb-tree for a multiset where I could use leafs as key 
counts. This is based on code by Alex Shinn which uses SRFI-146 which I 
had trouble using. I thought rb-tree could be useful here.

I noticed from reviewing the /src/runtime/rbtree.scm an undocumented 
function called rb-tree/update! which takes two optional arguments.

The function invokes apply-fail as follows (apply-fail fail 
'rb-tree/update!) but apply-fail is defined below with three arguments 
(apply-fail fail key caller).

Could it be passing the key is missing from rb-tree/update! ?

If yes, how can I fix this myself for my local version of scheme? I 
tried making the change and running Setup.sh / configure / make / make 
install with no luck.

Could you show me an example of how to call this function so that I can 
handle the case of a new key setting its value count to 1?

TIA

JP