def vcat(y)
      unless self.dims[1] == y.dims[1]
        raise ArgumentError, "Matrices must have same number of columns"
      end
      DoubleMatrix.concat_vertically(self, y)
    end
def vcat(y)
      unless self.dims[1] == y.dims[1]
        raise ArgumentError, "Matrices must have same number of columns"
      end
      DoubleMatrix.concat_vertically(self, y)
    end
Return a new matrix which consists of the _self_ on top of _y_.
 In general the hcat methods should be used sparingly. You
 should always ask yourself if an array of vectors or matrices
 doesn't serve you better. See also hcat.