docs: def completeness(self, mags, method='step'):
          """
          Return the completeness as a function of magnitude.
  
          ADW: Eventually want a completeness mask to set overall efficiency.
          """
          if method == 'step':
              func = lambda x: (x < self.mask_roi_unique[:,np.newaxis]).astype(float)
          elif method == 'erf':
              # Trust the ERD???
              # 95% completeness: 
              def func(x):
                  # Efficiency at bright end (assumed to be 100%)
                  e = 1.0
                  # SDSS EDR says full width is ~0.5 mag
                  width = 0.2 
                  # This should be the halfway point in the curve
                  maglim = self.mask_roi_unique[:,np.newaxis]
                  return (e/2.0)*(1/np.sqrt(2*width))*(np.sqrt(2*width)-scipy.special.erf((x-maglim)))
          else:
              raise Exception('...')
  
          return func(mags)