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().
Se 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 mus call full build, i.e. Vect_build(), at the end.

All features without category are written to category index of field 0. 
In category index is stored also type of feture, that make s possible to find areas
without centroid throug category index.