two test files demonstrating that eXist-2.1 seems to treat 'xquery' as a reserved namespace prefix in an imported module

ns_main.xq

xquery version "3.0";
import module namespace test="testimport" at "ns_module.xq";
declare namespace xquery="test";

declare variable $xquery:test := 'test';

test:echo($xquery:test)

ns_module.xq

xquery version "3.0";
(: in an imported module, eXist complains about syntax errors when 'xquery' is declared as the module's namespace prefix
 : this suggests that eXist in this case treats 'xquery' as reserved namespace prefix 
 :)
module namespace xquery="testimport";

declare function xquery:echo($string) {
    $string
};