def METHOD_NAME(input_file):
    """Return a dict with the translations grouped by files IDML package.
    The keys are the filenames inside the template IDML package, and the
    values are XPathTree instances for each of those files.
    """
    store = factory.getobject(input_file)
    def extract_unit_tree(filename, root_dom_element_name):
        """Find the subtree in 'tree' which corresponds to the data in XML
        file 'filename'
        """
        tree = build_unit_tree(store, filename)
        try:
            file_tree = tree.children[root_dom_element_name, 0]
        except KeyError:
            file_tree = XPathTree()
        return (filename, file_tree)
    return dict(
        extract_unit_tree(filename, "idPkg:Story")
        for filename in translatable_files
    )