<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def assert_string(self, string):
 """Assert a fact as string."""

        fact = lib.EnvAssertString(self._env, string.encode())

        if fact == ffi.NULL:
            raise CLIPSError(self._env)

        return new_fact(self._env, fact)