esent function calls.</p>
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<p>Predicate expressions can be created via frontends which can have a variety of forms.  The canonical expression of an expression, however, is via an s-expression (sexpr).  E.g.,</p>
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<pre>(and (gt 1000 (length (var 'Content-Length'))) (eq 'GET' (var 'Request-Method')))</pre>
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<p>Each parenthetical subexpression represents a function call with the name of the