def METHOD_NAME(self, filename="envvars"):
    """Save envvars needed for CNS to a file in the module's folder."""
    # there are so few variables, best to handle them by hand
    lines = (
        "#!/bin/bash",
        "# for debugging purposes source this file from within the ",
        "# module folder for example, from within '00_topoaa'",
        "export MODULE=cns",
        "export MODDIR=.",
        "export TOPPAR=../toppar",
        )
    fstr = os.linesep.join(lines)
    Path(self.path, filename).write_text(fstr)
    return