The absence of an example in the spec doesn't mean that something can't be done. (The XPath expression //.. never appears in the spec, but it is perfectly legal and even potentially useful). But yes. + and - are operators (not "primitives"), and operators are not functions. You can easily wrap a function around an operator: function($x, $y) {$x + $y} and pass this function to higher-order functions such as fold-left, but you cannot pass the operator itself. Michael Kay Saxonica
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