CondExpr...

is an expression parser. You use it via the "expr" markup, thusly:

   (:if expr equal {$var1} {$var2} xor equal {$var3} {$var4}) 

   (:if expr ! equal 'carrot' {$var4})

Operations available are:

I think you can also use parenthetical expressions like

(: if expr ({$bool1} or {$bool2}) and ({$bool3} or {$bool4}) :)

Usage examples here:

ConditionalMarkup - The if directive allows portions of a page to be included or excluded from rendering
Cookbook:JJSApprovePage -

This page may have a more recent version on pmwiki.org: PmWiki:CondExpr, and a talk page: PmWiki:CondExpr-Talk.