code: def add_particle(
          self,
          pos=(0, 0, 0),
          charge=1e-6,
          mass=1e-3,
          radius=0.005,
          color=None,
          vel=(0, 0, 0),
          fixed=False,
          negligible=False,
      ):
          
          color = color or len(self.particles)  # assigned or default color number
  
          p = Particle(pos, charge, mass, radius, color, vel, fixed, negligible)
          self.particles.append(p)
docs: Adds a new particle with specified properties (in SI units)