'Create a TranslationUnit instance from a saved AST file.
A previously-saved AST file (provided with -emit-ast or
TranslationUnit.save()) is loaded from the filename specified.
If the file cannot be loaded, a TranslationUnitLoadError will be
raised.
index is optional and is the Index instance to use. If not provided,
a default Index will be created.'

@classmethod
def from_ast_file(cls, filename, index=None):

    if (index is None):
        index = Index.create()
    ptr = conf.lib.clang_createTranslationUnit(index, filename)
    if (not ptr):
        raise TranslationUnitLoadError(filename)
    return cls(ptr=ptr, index=index)
