def collect_matrix
      return to_enum(:collect_matrix) unless block_given?

      vecs = vectors.to_a
      rows = vecs.collect { |row|
        vecs.collect { |col|
          yield row,col
        }
      }

      Matrix.rows(rows)
    end
Generate a matrix, based on vector names of the DataFrame.

@return {::Matrix}
:nocov:
FIXME: Even not trying to cover this: I can't get, how it is expected
to work.... -- zverok