              The mean value of the figure of merit m for reflections for this\
               derivative in this shell.\
\
                   int P~alpha~ exp(i*alpha) dalpha\
               m = --------------------------------\
                         int P~alpha~ dalpha\
\
               P~alpha~ = the probability that phase angle alpha is correct\
\
               int is taken over the range alpha = 0 to 2 pi.