code: def _gradient_rgb_line(
              self, text, start, stop, step=1,
              fore=None, back=None, style=None):
          
          return self._gradient_rgb_line_from_morph(
              text,
              list(self._morph_rgb(start, stop, step=step)),
              fore=fore,
              back=back,
              style=style
          )
docs: Yield colorized characters, morphing from one rgb value to
              another.