Along this overview I have used the tensorial calculus from [87, 21, 36], with the appropriate gauge to have simpler expressions. Instead of reproducing here the concepts involved in this calculus I'm going to express in words the geometric meaning of the symbols that appear along the overview, which, I think, is a more practical solution in this case. We have the following link between 'symbols' and 'meaning':
being 
and 
unit vectors. Therefore
In the 2-dimensional case we have
In d dimensions, expressions generalize according to the tensorial calculus in [87, 21, 36].