code: def custom_to_radec(phi1,phi2,T=None,degree=False):
      
      if T is None: raise ValueError("Must set T= for custom_to_radec")
      return radec_to_custom(phi1, phi2,
                             T=nu.transpose(T),  # T.T = inv(T)
                             degree=degree)
docs: NAME:
  
          custom_to_radec
  
      PURPOSE:
  
         rotate a custom set of sky coordinates (phi1, phi2) to (ra, dec)
         given the rotation matrix T for (ra, dec) -> (phi1, phi2)
  
      INPUT:
  
          phi1 - custom sky coord
  
          phi2 - custom sky coord
  
          T - matrix defining the transformation (ra, dec) -> (phi1, phi2)
  
          degree - default: False. If True, phi1 and phi2 in degrees
  
      OUTPUT:
  
          (ra, dec) for vector inputs [:, 2]
  
      HISTORY:
  
          2018-10-23 - Written - Nathaniel (UofT)