I'm looking to implement extension instructions much like I've implemented extension functions.  I'd like to:
1 - provide my users with a URI that represents a set of functions and instructions - with no mention of a class name. 
2 - prevent a user from plugging in their own java code as an ExtensionElementFactory.
3 - create the instance of the extension instruction myself, instead of Saxon doing so reflectively. 
Essentially, I'm looking for a corollary of FunctionLibrary that I can set on the Configuration object.  Is there some underlying reason for the disparity between extension functions and extension instructions?  Is there a way to accomplish what I'm looking to do?

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