auxiliary classes; may contain either a vector or NULL, [or a matrix, or an array, respectively], cf. J. Chambers, "green book".
these classes are all virtual: No objects may be created from it.
No methods defined with classs "vectororNULL", "MatrixorNULLorVector", and
"ArrayorNULLorVectororDataframeorSeqDataFrames" in the signature.
However, the generating function Dataclass dispatches according to
"DataframeorSeqDataFrames" or "ArrayorNULLorVector".