new octave_value::cell_value method with optional error message * ov.h, ov.cc (octave_value::cell_value): New method. * ov-base.h, ov-base.cc (octave_base_value::cell_value): New default method. * ov-cell.h, ov-cell.cc (octave_cell::cell_value): New method.

function h = plot (p, varargin)
  n = 128;
  rmax = max (abs (roots (p.poly)));
  x = [0 : (n - 1)] / (n - 1) * 2.2 * rmax - 1.1 * rmax;
  if (nargout > 0)
    h = plot (x, p(x), varargin{:});
  else
    plot (x, p(x), varargin{:});
  endif
endfunction