<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _check_nan(self, epoch_data: EpochData) -> None: """ Raise an exception when some of the monitored data is NaN. :param epoch_data: epoch data checked :raise KeyError: if the specified variable is not found in the stream :raise ValueError: if the variable value is of unsupported type and ``self._on_unknown_type`` is set to ``error`` """

        for stream_name in epoch_data.keys():
            stream_data = epoch_data[stream_name]
            variables = self._variables if self._variables is not None else stream_data.keys()
            for variable in variables:
                if variable not in stream_data:
                    raise KeyError('Variable `{}` to be nan-checked was not found in the batch data for stream `{}`. '
                                   'Available variables are `{}`.'.format(variable, stream_name, stream_data.keys()))

                value = stream_data[variable]
                if self._is_nan(variable, value):
                    raise TrainingTerminated('Variable `{}` is NaN.'.format(variable))