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This is the description of the 'query' paramter:
$query: A query string component, if any.
Due to the fact that the value is simply handed over to the url() function, the description should be the same as on page http://api.drupal.org/api/drupal/includes!common.inc/function/url/6
'query': A URL-encoded query string to append to the link, or an array of query key/value-pairs without any URL-encoding