Just got it! Thanks for your help!
I was using ams* files, so I went into those files and deleted the offending
theorem definitions. It now compiles just fine. (-:
Thanks again!
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How do I force LyX 1.5.2 to redefining the theorem environment?
I'm required to use stacs.cls as my document class (and am not allowed to
modify this file). Stacs.cls defines the theorem environment, and this
conflicts with LyX wanting to automatically add (another) theorem
environment. So, how