One of the fundamental features of i_labelled objects is that both
underlying values and value labels can be used for logical
operations.
The following rules must be observed when indexing i_labelled
objects:
If numerical values are used for the indexation of numerical
i_labelled data, the underlying numerical values of the i_labelled data
are used for the comparison.
If character values or values of class i_labelled are used for
indexation, value labels are always used over the underlying values
of the i_labelled data.