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					Originally Posted by cybmole  it was not write protected, I'd imported it into calibre library before trying to open it with sigil.i am not sure what I'd be checking for, within tweak epub ?
 
 is a file called meta.inf a mandatory component, but one which ADE & calibre viewer can live without ?
 
 PS a calibre conversion did repair the book , as expected.
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 Yes
There are 3 sections to the spec (see at) 
http://idpf.org/epub