Find the asymmetry of each helicity.

def _get_asym_hel(self,d):
        """
            Find the asymmetry of each helicity. 
        """
        
        # get data 1+ 2+ 1- 2-
        d0 = d[0]; d1 = d[2]; d2 = d[1]; d3 = d[3]

        # pre-calcs
        denom1 = d0+d1; 
        denom2 = d2+d3
        
        # check for div by zero
        denom1[denom1==0] = np.nan          
        denom2[denom2==0] = np.nan
        
        # asymmetries in both helicities
        asym_hel = [(d0-d1)/denom1,
                    (d2-d3)/denom2]
                    
        # errors 
        # https://www.wolframalpha.com/input/?i=%E2%88%9A(F*(derivative+of+((F-B)%2F(F%2BB))+with+respect+to+F)%5E2+%2B+B*(derivative+of+((F-B)%2F(F%2BB))+with+respect+to+B)%5E2)
        asym_hel_err = [2*np.sqrt(d0*d1/np.power(denom1,3)),
                        2*np.sqrt(d2*d3/np.power(denom2,3))]
        
        # remove nan            
        for i in range(2):
            asym_hel[i][np.isnan(asym_hel[i])] = 0.
            asym_hel_err[i][np.isnan(asym_hel_err[i])] = 0.
        
        # exit
        return [[asym_hel[1],asym_hel_err[1]],  # something wrong with file?
                [asym_hel[0],asym_hel_err[0]]]