shouldn't plain old !if work also? because that doesnt work. With your syntax change I get a problem:
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Exception during bang command execution.
Please contact the module author
Bang parameters: "no" [!msgbox "message"]
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[link]http://www.leechprint.com/phil/Readmes/mzscript.htm[/link] also suggests that there is nothing wrong with the previous syntax, although it is for an earlier version.