About the gluon feild and propagator

1. Aug 9, 2014

ccnu

I am confused about the very step from Eq.(14) to Eq.(15) in the paper
http://arxiv.org/pdf/hep-ph/0308157.pdf
There is one $ig$ in (14), while $(ig)^2$ in (15), where does it come from?
Regards!

2. Aug 13, 2014