code: def tetrad(clr, angle=90):
      
      clr = color(clr)
      colors = colorlist(clr)
  
      c = clr.rotate_ryb(angle)
      if clr.brightness < 0.5:
          c.brightness += 0.2
      else:
          c.brightness -= -0.2
      colors.append(c)
  
      c = clr.rotate_ryb(angle * 2)
      if clr.brightness < 0.5:
          c.brightness += 0.1
      else:
          c.brightness -= -0.1
      colors.append(c)
  
      colors.append(clr.rotate_ryb(angle * 3).lighten(0.1))
  
      return colors
docs: Returns a tetrad of colors.
  
      The tetrad is made up of this color and three other colors
      that together make up a cross on the artistic color wheel.