--- trunk/dyn3d/principal_cshift.f	2015/02/05 15:19:37	124
+++ trunk/dyn3d/principal_cshift.f	2015/02/06 15:00:28	125
@@ -6,6 +6,10 @@
 
   subroutine principal_cshift(is2, xlon, xprimm)
 
+    ! Add or subtract 2 pi so that xlon is near [-pi, pi], then cshift
+    ! so that xlon is in ascending order. Make the same cshift on
+    ! xprimm.
+
     USE dimens_m, ONLY: iim
     use nr_util, only: twopi
     use serre, only: clon