[cairo] CAIRO_OPERATOR_XOR documentation

[Richard Billington]

If I interpret this correctly (quoting from 
http://cairographics.org/operators/):

    Resulting alpha (aR)    Resulting color (xR)
    aA + aB − 2·aA·aB    (xaA·(1−aB) + xaB·(1−aA))/aR

If the alpha value for both A and B is 1, then nothing will be drawn 
anywhere? That is, I have something drawn on surface A with alpha at 
1.0, and I have something on the receiving surface drawn with alpha at 
1.0. This results in aR = 0.0 so everything in B becomes transparent?
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