def _draw_particle(self, pos, sign=<NUM_LIT:1>):
raise NotImplementedError('<STR_LIT>')<EOL>
Updates ``self.particles`` by drawing a particle at position ``pos``,
with possible additional unnamed arguments between ``pos`` and
``sign``. If ``sign`` is -1, un-draws the particle instead.

To be able to fit this component in a model, _draw_particle must
create an image that is numerically continuous as pos changes --
i.e. the edge of the particle must alias smoothly to 0.