How to precisely locate every circles in a image using simpleCV

I am trying to solve a image processing problem with SimpleCV. How to precisely locate every circles? Now I can only detect the number of the circles, which is 24. Then I need to give each circle a number, A1...A6. B1...B6. C1...C6. D1...D6 like the following pattern.

1   2   3   4   5   6
0   0   0   0   0   0  A
0   0   0   0   0   0  B
0   0   0   0   0   0  C
0   0   0   0   0   0  D


When I cover 2 circles, how to identify who they are, such as A2 and C3 are covered. when covering more, it can also tell what the numbers of the disappeared circles. Can anyone give me some brief instruction? Thank you very much!

from SimpleCV import Image, Camera, Display
cam = Camera()
display = Display()
img = Image("circles.jpg")
circles = img.findCircle (canny=70, thresh=200)
circles.show()
print(len(circles))

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