The S4Vectors package defines the Vector and List virtual classes
and a set of generic functions that extend the semantic of ordinary
vectors and lists in R. Package developers can easily implement
vector-like or list-like objects as concrete subclasses of Vector or
List. In addition, a few low-level concrete subclasses of general
interest (e.g. DataFrame, Rle, and Hits) are implemented in the
S4Vectors package itself (many more are implemented in the IRanges
package and in other Bioconductor infrastructure packages).