The function creates a temporary local variable, whose 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 expressions, 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.
Boolean
This expression returns true if all reports have positive incomes recorded.
every('income_value', /report/income, $income_value > 0)