title: local variable refernced before assigement error
body: <p>I am very new to codding and am trying to make a game I can call my own. I don't undertand what the error means but here is my code:</p>
  
  <pre><code>import pgzrun
  import math
  import random
  
  WIDTH = 800                       #5
  HEIGHT = 600
  CENTER_X = WIDTH/2
  CENTER_Y = HEIGHT/2
  CENTER = (CENTER_X, CENTER_Y)
  FONT_COLOR = (221, 160, 221)      #10
  ozol = 0
  
  game_over = False
  
  ozolith = Actor("ozolith")        #15
  
  def draw():
      screen.clear()
      ozolith.draw()
      draw_counters(ozol)           #20
  
  def place_ozolith():
      ozolith.x = CENTER_X
      ozolith.y = CENTER_Y
                                    #25
  def draw_counters(ozol):
      screen.draw.text(str(ozol), fontsize=40, center=CENTER, color=FONT_COLOR)
  
  def on_mouse_down(pos):
      if ozolith.collidepoint(pos): #30
          ozol += 1
          place_ozolith
      else:
          quit()
                                    #35
  place_ozolith
  
  pgzrun.go()
  </code></pre>
  
  <p>Here is the error message </p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Program Files\Python38\python-games\GameCraft\GameCraft.py", line 38, in &lt;module&gt;
      pgzrun.go()
    File "C:\Program Files\Python38\lib\site-packages\pgzrun.py", line 31, in go
      run_mod(mod)
    File "C:\Program Files\Python38\lib\site-packages\pgzero\runner.py", line 113, in run_mod
      PGZeroGame(mod).run()
    File "C:\Program Files\Python38\lib\site-packages\pgzero\game.py", line 217, in run
      self.mainloop()
    File "C:\Program Files\Python38\lib\site-packages\pgzero\game.py", line 247, in mainloop
      self.dispatch_event(event)
    File "C:\Program Files\Python38\lib\site-packages\pgzero\game.py", line 172, in dispatch_event
      handler(event)
    File "C:\Program Files\Python38\lib\site-packages\pgzero\game.py", line 164, in new_handler
      return handler(**prepped)
    File "C:\Program Files\Python38\python-games\GameCraft\GameCraft.py", line 31, in on_mouse_down
      ozol += 1
  UnboundLocalError: local variable 'ozol' referenced before assignment
  </code></pre>
  
  <p>It happens once I click on the image which is stuck on the top left corner.</p>