Type Alias FunctionLookupFn 

pub type FunctionLookupFn = Box<dyn Fn(&XPathContext, &str) -> Option<BoxedXPathFunction> + Send + Sync>;

A namespaced function-lookup fallback: consulted by lookup_function after the exact-name registry misses, so engines (XSLT) can resolve prefix:local(...) calls against their own extension registries.

Aliased Type

pub struct FunctionLookupFn(/* private fields */);