Saxon

OK, it calls function-lookup to get {http://mathling.com/art/backgrounds}algorithm-parameters#2. The current package is http://mathling.com/art/components. And it fails because the current package knows nothing of such a function.

The main package uses both components and backgrounds, but neither of these uses each other.

The spec for fn:function-lookup says that it looks "in the named functions component of the dynamic context "

The spec for the dynamic context in XSLT (§5.3.3.2) says this includes all the functions in the static context (plus an implementation-defined set of functions that are available dynamically but not statically.) The spec for the static context says that it includes

• The stylesheet functions defined in the containing package;
• Stylesheet functions defined in used packages, subject to visibility: see 3.5.2 Dependencies between Packages;

So the static context doesn't include the desired function, and therefore neither does the dynamic context.

So it's basically failing by design. It's trying to ensure that you can't use function-lookup to get around the visibility rules; you can't have a run-time dependency on a package that you didn't declare a compile-time dependency on.

My first though was that it would work if you declared the function with visibility="public". But that's not the problem (you actually expose it as public): you have to declare a "use-package" relationship as well.

Closing the loop: this is exactly what I did, and it works a treat. Thanks for the clarification.