+ A huge optimization is possible in insert_idml() in reusing the same working copy in the several methods.
+ Add an abstraction over "working_copy_path" when working in mode="w" on IDMLPackages. This should be an IDMLPackage 
    subclass ? Or maybe a method to push/synchronize an IDMLPackage modifications (stored in RAM with Spreads, Pages etc)
    to the working copy.
+ idml.XMLDocument is shit. Should be replace by IDMLXMLFile and subclasses like Spread etc.
+ In insert_idml() and add_page(), only add mandatory Stories, not all of them.


tests
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BackingStory.get_root()
Tags()
Fonts()
Style.style_groups()
