How does a processor get selected?

Suppose I¹m using Calabash, and it has its own XSLT processor (engine).
Suppose I later decide I want to use a different XSLT processor, like Saxon.
Does XProc itself allow me to select which processor to use?  Or is that
implementation defined?  Or is that even an option?

TIA,

James Garriss
http://garriss.blogspot.com