INCUCU ( cu1, bound1, cu2, bound2, surf, intbox, intpts, ipars1, ipars2, 
=========================================================================
         incods, nintpt, ifail )
         =======================

  Intersect two curves

Receives:
  KI_tag_curve             *cu1            --- curve 1
  KI_vec_position           bound1[2]      --- start and end of curve 1
  KI_tag_curve             *cu2            --- curve 2
  KI_vec_position           bound2[2]      --- start and end of curve 2
 <KI_tag_surface>          *surf           --- surface containing both curves
  KI_dbl_box                intbox[6]      --- box of interest

Returns:
 <KI_tag_list_dbl>         *intpts         --- points of intersection
 <KI_tag_list_dbl>         *ipars1         --- parameters of intersections
                                           --- on 'cu1'
 <KI_tag_list_dbl>         *ipars2         --- parameters of intersections
                                           --- on 'cu2'
 <KI_tag_list_int>         *incods         --- tokens describing intersections
 <KI_int_nitems>           *nintpt         --- number of points returned
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_cant_do_intersect      cant perform requested intersection
                            cant do intersect
  KI_not_on_surface         curve is not on the given surface
  KI_invalid_geometry       curve fails checks
                            surface fails checks
  KI_not_on_curve           start or end point not on curve
  KI_bad_end_points         curve not defined between start and end points

Description:

  INCUCU finds the intersections between specified regions of two curves.
  It returns a list of intersection coordinates, two lists containing the
  parameters of the curves at the intersections, and a list of tokens
  classifying the intersections. The intersections are ordered along the
  first curve, 'cu1', and are classified according to the direction of this
  curve.

  The intersection coordinates are returned in a list of length 3*nintpt; if
  there are no intersections of the curves within the bounds, 'nintpt' is
  returned as zero and 'intpts' is returned as the NULTAG.

  The parameters of 'cu1' at the intersections are returned as a list of length
  'nintpt', as are the parameters of 'cu2'.

  The intersection types are returned in a list of length 'nintpt'.  There are
  three types of intersection, given by the tokens CICLSI, CICLSC, and CICLEC.

      CICLSI:  A simple intersection not adjoining a region of coincidence.
               This includes tangent intersections.
      CICLSC:  An intersection at the start of a region of coincidence.
      CICLEC:  An intersection at the end of a region of coincidence.

      Whether an intersection is at the start or end of a region of coincidence
  (CICLSC or CICLEC) is determined by the direction of the first curve, 'cu1',
  passed to INCUCU.

  Coincident intersections (CICLSC and CICLEC) are returned for all points
  lying at the bounds of regions of coincidence, including the ends of fully
  coincident curves (or regions of curves).  In the case where two closed curves
  are coincident, but the bounds of 'cu1' are not coincident with those of
  'cu2', the intersections returned will be at the bounds of 'cu1'.

  The regions of the curves which are to be intersected are specified
  by giving start and end coordinates on each. These are given in the
  arrays 'bound1' and 'bound2', in the order start and then end. If these
  points are the same, and the geometry of the curve is closed, the
  complete curve will be used. For a trimmed curve the supplied bounds are
  ignored. However valid vectors should be supplied for the curve in question.

  A surface may be supplied which contains both the curves, otherwise it
  should be NULTAG. The function will work without this surface but it may
  be more efficient to supply it.

  'intbox' describes the box that contains the area of interest.  All
  intersections inside this box will be returned, but ones outside it may
  not be.  The box may be used for efficiency.  It 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.

  Any B-curve, B-surface or offset 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).