environment-find-function (environment local-name uri)
an XPath function or nil if it cannot be found
Finds an XPath function by local-name and uri.
XPath function is a Lisp function that takes zero or more "thunks" as its arguments (corresponding to XPath expressions passed as function arguments) and returns a new "thunk". A "thunk" is a function that takes an instance of context as its argument and returns the value of one of XPath types.