index-class: Virtual Class "index" - Simple Class for Matrix Indices
Description
The class "index" is a virtual class used for
  indices (in signatures) for matrix indexing and sub-assignment of
  "Matrix" matrices.  In fact, it is currently implemented as a simple class union
  (setClassUnion) of
  "numeric",  "logical" and "character".
Objects from the Class
Since it is a virtual Class, no objects
  may be created from it.