external functions in xquery

Are there any limits imposed on external functions in XQuery?  E.g., can
external functions have side-effects which might affect documents referenced
in the query, in which case evaluation order is important?  Must the value
returned by an external function for a given set of parameters be
independent of when that function call is made, or the number of such
function calls?

 

(I notice there is an editorial note in the current formal semantics draft
which might cover these questions, which references "Issue 522
(FS-Issue-0179)".)

 

Similarly, for external variable declarations, must the value of the
variable remain constant?  I personally think this is a reasonable
restriction.

 

In my opinion, keeping XQuery purely declarative would be an important goal,
and so I hope that the restrictions on external functions and variables
would reflect that.