@classmethod<EOL><INDENT>def register(cls):<DEDENT>
if not cls.IS_LOADED:<EOL><INDENT>cls.IS_LOADED = True<EOL>yaml.add_constructor('<STR_LIT>', Parameter.parameter_constructor, Loader=yaml.SafeLoader)<EOL>yaml.add_constructor('<STR_LIT>', EnvironmentVariable.parameter_constructor, Loader=yaml.SafeLoader)<EOL><DEDENT>
Register variable handling in YAML