From what I gathered, "ExecLib" is a command that invokes a hook that calls a special FlashAPP. The thing you're supposed to use before that command is "OpenLib(" which contains the name of the application you want to open up for the ExecLib command. The problem with that is that the FlashAPP has to have something special in its header for that to work, so only applications written for that functionality will be able to use it.
More information on that is in the WikiTI.
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DJ Omnimaga: I've forgotten exactly what those problems were with Celtic III's xLIB emulation. Could you repost what problems you've experienced were? I lost that list of potential issues since the last time you told me about them >.<