code: def _gradient_black_lines(
              self, text, start, step=1,
              fore=None, back=None, style=None, reverse=False,
              movefactor=2, rgb_mode=False):
          
          if not movefactor:
              def factor(i):
                  return start
          else:
              # Increase the start for each line.
              def factor(i):
                  return start + (i * movefactor)
          return '\n'.join((
              self._gradient_black_line(
                  line,
                  start=factor(i),
                  step=step,
                  fore=fore,
                  back=back,
                  style=style,
                  reverse=reverse,
                  rgb_mode=rgb_mode,
              )
              for i, line in enumerate(text.splitlines())
          ))
docs: Yield colorized characters,
              within the 24-length black gradient,
              treating each line separately.