CRINCU ( surf1, surf2, intbox, curves, ncurve, ifail )
======================================================

  Create intersection curves.

Receives:
  KI_tag_surface           *surf1          --- surfaces to be
  KI_tag_surface           *surf2          --- intersected
  KI_dbl_box                intbox[6]      --- box of interest

Returns:
 <KI_tag_list_curve>       *curves         --- list of curves
 <KI_int_nitems>           *ncurve         --- number of curves returned
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_cant_do_intersect      failure in intersection routine
  KI_su_are_coincident      coincident surfaces
  KI_dont_intersect         no intersection
  KI_invalid_geometry       surface fails checks

Description:
  New curves are created wherever the two surfaces intersect (if they do).
  The list of new curves is returned in 'curves'.  All curves which pass
  through the box of interest will be returned; curves which lie completely
  outside the box may be returned.  Curves may be truncated more or less
  immediately when they leave the box, and a curve which leaves and reenters
  the box may be returned as several disjoint sections.

  'intbox' describes the box that contains the area of interest and is
  specified as follows.

  Its 1st element contains the minimum x_component of the area of interest
  "   2nd    "        "     "  minimum y_component  "  "    "  "     "
  "   3rd    "        "     "  minimum z_component  "  "    "  "     "
  "   4th    "        "     "  maximum x_component  "  "    "  "     "
  "   5th    "        "     "  maximum y_component  "  "    "  "     "
  "   6th    "        "     "  maximum z_component  "  "    "  "     "

  For a box to be valid the difference between the maximum and minimum
  components in all three principal directions must be greater than or
  equal to zero.

  If either surface has type TYSUPA (B-surface) then that surface must be
  capable of passing the checks imposed by CHCKEN.

  The self intersection check is only performed if the appropriate option is set
  (see SEINTP).