This function is used by memory classes to select the output value
to use for the memory. If there is only one output value, it is used.
If there are multiple output values, the outputKey must be specified.
If no outputKey is specified, an error is thrown.
This function is used by memory classes to select the output value to use for the memory. If there is only one output value, it is used. If there are multiple output values, the outputKey must be specified. If no outputKey is specified, an error is thrown.