docs: def set_bfield(self, B_G):
          """Set the strength of the local magnetic field.
  
          **Call signature**
  
          *B_G*
            The magnetic field strength, in Gauss
          Returns
            *self* for convenience in chaining.
          """
          if not (B_G > 0):
              raise ValueError('must have B_G > 0; got %r' % (B_G,))
  
          self.in_vals[IN_VAL_B] = B_G
          return self