Corrected inclination, taking into account backsight and clino corrections.

def inc(self):
        """Corrected inclination, taking into account backsight and clino corrections."""
        inc1 = self.get('INC', None)
        inc2 = self.get('INC2', None)
        if inc1 is None and inc2 is None:
            return None
        if inc2 is None:
            return inc1
        if inc1 is None:
            return -1 * inc2
        return (inc1 - inc2) / 2.0