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.