specifying text node filtering rules

Hello! I wonder if the following is possible and cannot find an answer
to it. I use XQuery code to read an input XML document "in.xml" and
produce an output XML document "out.xml". I wonder if I can specify a
rule so that all text nodes copied from "in.xml" to "out.xml" are
filtered as follows: every group of consecutive whitespace characters
(tabs, new lines, spaces) are replaced by a single space character.
Given that text nodes are copied by many, diverse ways in my code, I
am not looking for a function manipulating strings (that should be
called whenever needed); this, apart from being a hardcoded procedure,
wouldn't be enough when copying element nodes containing text nodes in
their tree structures. Instead, I am looking for a way to specify a
general rule for filtering text nodes (e.g. like "declare
boundary-space strip;").

Thanks in advance.