I'm reading the excellent picamera documentation (thank you so much!) and I have now a better idea of how the sensor works. I'd like to perform some RAW Bayer data captures myself, so began reading section 4.5 Raw Bayer data captures

At some point, the example code states that the OV5647 sensor Bayer pattern is BGGR and then, it shows the equivalent
pixel CFA as

GBGBGBGBGBGBGB
RGRGRGRGRGRGRG
GBGBGBGBGBGBGB
RGRGRGRGRGRGRG

I simply do not get how the BGGR pattern is deducted from the layout.

For instance, reading 2x2 from top, left I would read GBRG
For instance, reading 2x2 from bottom, left I would read RGGB

I would be very grateful if you could expand the comments so that the BGGR pattern can be easily seen from the layout depicted.