
The function creates a temporary local variable, whise name is specified by
varname argument.
Then, for every item of test_set sequence it
calculates the test_expn boolean expression
having set the created variable to that "current" item.
If the value of expression is false, the function immediately returns
false without processing the rest of test_set sequence.
If all items of the sequence are probed without getting false,
true is returned. (So if the sequence is empty, the function returns true).

In any case, temporary variable is destroyed on return.

This function is used in the implementation of
"EVERY" logical operator in XQUERY, so you will probably use that operator
in XQUERY expessions, not the function.
This function may be useful in XPATH expressions and in XSLT stylesheets.
It is not a part of library of standard XQUERY 1.0 functions.
