def to_string(self, fmt: str = "<STR_LIT>") -> str:
return f"<STR_LIT>"<EOL>
Convert AST object to string

        Args:
            fmt (str): short, medium, long formatted BEL statements
                short = short function and short relation format
                medium = short function and long relation format
                long = long function and long relation format

        Returns:
            str: string version of BEL AST