Dangles
  ---------
GRASS doesn't allow areas/islands with dangles. If an island has dangle, it is not built
and attached to outer area and as a consequence, some functions do not work as expected.
For example Vect_area_area() and Vect_find_area().
See comments in build_nat.c: Vect_attach_centroids()

   Category index
  ----------------
Category index is created when Vect_build(), i.e. Vect_build_partial(, GV_BUILD_ALL,) 
is called. Plus_head.cidx_up_to_date is set to 1 if Category index is created.
Any later call to Vect_write_line() / Vect_rewrite_line() / Vect_delete_line() resets
Plus_head.cidx_up_to_date to 0 and doesn't update category index. 
All modules using update on level 2 must call full build, i.e. Vect_build(), at the end.

All features without category are written to category index of field 0. 
The category index also stores type of feature, that makes it possible to find areas
without centroid through category index.