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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def declaration_files(decl_or_decls):
 """ Returns set of files Every declaration is declared in some file. This function returns set, that contains all file names of declarations. :param decl_or_decls: reference to list of declaration's or single declaration :type decl_or_decls: :class:`declaration_t` or [:class:`declaration_t`] :rtype: set(declaration file names) """


    files = set()
    decls = make_flatten(decl_or_decls)
    for decl in decls:
        if decl.location:
            files.add(decl.location.file_name)
    return files