SETLEN ( entity, tol, edges, nedges, ifail )
============================================

  Associates a tolerance value with a face, edge or vertex.

Receives:
  KI_tag_entity            *entity         --- can be face, edge or vertex
  double                   *tol            --- new tolerance value

Returns:
 <KI_tag_list_edge>        *edges          --- list of new edges
  KI_int_nitems            *nedges         --- number of edges
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_bad_tolerance          new tolerance smaller than old
                            edge is too short for supplied tolerance
                            tolerance is less than half Parasolid tolerance
  KI_tolerances_too_tight   SP-curve generator could not achieve tol
  KI_failed_to_create_sp    SP-curve generator failed - unknown reason
  KI_missing_geom           neither adjacent face has a surface
                            edge has no curve
                            face has no surface attached
  KI_general_body           General body
  KI_wrong_entity           unsuitable entity type

Description:
  SETLEN alters the tolerance value associated with the given entity, which is
  allowed to be a face, edge or vertex. The tolerance is a resolution
  parameter, and represents the minimum distance (in 3-space) that a point and
  'entity' must be apart in order to be regarded as distinct. It reflects the
  user's belief about the accuracy of the geometric data in the body.

  The action of SETLEN in each case is as follows:

  FACE - Associating a tolerance value with a face indicates that its surface
         is to be viewed as having a region of uncertainty around it whose
         extent is given by the tolerance. Attempting to set a tolerance on a
         face without a surface will result in an error. Currently face
         tolerances are not used internally.

  EDGE - The action for edges is more complicated, and depends on whether the
         edge previously had Parasolid tolerance associated with it or not
         (i.e. whether Parasolid regarded the edge as "exact").

         The action for exact edges is as follows:

         The 3-space edge curve is used to create SP-curves on the adjacent
         surfaces, which are subsequently attached to the fins of the edge.
         These will be stored along with their bounding information as
         Trimmed Curves (type TYCUTR). The tolerance of the edge is to be
         viewed as a band of uncertainty within which all defining SP-curves
         lie.
         There will always be the following side-effects:
         . The original 3-space curve will be detached and deleted if possible.
         . The tolerances of all vertices on the edge will be increased if
           necessary so as to be greater than or at least equal to 'tol'.
           The point on the vertex will be recomputed.
         . All edges meeting at any vertex of the given edge which possess
           3-space unbounded curves will have these converted to Trimmed
           Curve form, with the original curve as the basis curve. This is
           because the assumption that the vertex point lies on all edge
           curves meeting there will no longer hold true.
         . It will be impossible to make the edge "exact" again by using
           SETLEN. Once this routine has been used on an edge, it will continue
           to be represented geometrically by means of SP-curves attached to
           the fins unless MENDEN or RETLEN is called.
         Additionally, the following may occur:
         . It may not always be possible to compute one SP-curve from the
           original curve and the relevant face surface. This will be due
           to G1 discontinuities (in the surface parameter space) which will
           have to be represented as extra vertices. The edge may therefore
           be split a number of times, once for every surface singularity or
           discontinuity which lay on the original curve. Any new edges created
           by this process will be returned to the user in 'edges'.

         It is an error to set a tolerance on an "exact" edge when the
         3-space curve is not present.

         It is an error if the edge is shorter than twice the given 'tol'.

         The action for edges already possessing a tolerance is as follows:

         The SP-curve(s) already present on the fins of the edge will be
         unchanged by SETLEN. The tolerance of the edge will be set to 'tol'.
         The tolerances of vertices of the edge will be increased if
         necessary so as to be greater than or at least equal to the supplied
         tolerance. It should be noted that reducing the tolerance value on
         an already toleranced edge is not recommended, as the existing
         SP_curve(s) will not be replaced in this case. Hence such an edge
         may fail CHCKEN.

  VERTEX - The tolerance of a vertex is to be viewed as the radius of a sphere
           of uncertainty around the point of the vertex. Setting the tolerance
           value is subject to the following restrictions:
           . it is not permitted to set the tolerance of a vertex to less than
             the maximum of the tolerances of the edges meeting at it.
           . attempting to set a tolerance on a vertex without a point
             attached will result in an error.

  Note that setting tolerances on faces, edges and vertices risks rendering
  the model invalid, as CHCKEN will use these tolerance values for
  determining whether geometric items meet appropriately or not. For example,
  increasing the tolerance of a particular vertex may result in its clashing
  with a nearby edge, or being within tolerance of another vertex. Both of
  these configurations will be recognised by CHCKEN. Some checks are made for
  local effects of this nature but such checking is by no means exhaustive.

  This function is not supported for entities which are part of a
  general body.