code: def PhenomModel(self, r):
          
          if r <= 0:
              raise ValueError
          field = self.B0 + self.B1 * G4.m / r + self.B2 * math.exp(-1 * self.H * r / G4.m)
          return field
docs: Fit to field map
  
          A phenomenological fit by Ryan Bayes (Glasgow) to a field map
          generated by Bob Wands (FNAL).  It assumes a 1 cm plate.  This is dated
          January 30th, 2012.  Not defined for r <= 0