k
A parameter from either the command function, or from an aggregation function.
If the aggregator is user-defined, then this parameter comes from the aggregation function.
If the aggregator was created from a single option (this one), then this parameter comes from the command function.
In other cases, the parameter comes from an aggregation function
This parameter is thus suitable for inspection.
Note: An aggregator parameter may be referenced by multiple options (varargs, for example).