RETLEN ( entity, retcod, ifail )
================================

  Restores Parasolid tolerance to the supplied edge.

Receives:
  KI_tag_entity            *entity         --- edge

Returns:
  KI_cod_rttl              *retcod         --- status of operation
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_wrong_entity           unsuitable entity type

Description:
  RETLEN attempts to remove the tolerance value associated with the given
  edge, restoring Parasolid default and replacing fin curves by a single
  3-space curve attached to the edge. This operation may not always be
  possible - the level of success is indicated by the status code 'retcod'
  which can take the following values:

       retcod value   |          status of operation
     ----------------------------------------------------------------
          RTTLOK      |  successfully replaced tolerance and geometry
                      |
          RTTLNT      |  neither tolerance nor geometry replaced as
                      |  surfaces are approximately tangent at edge
                      |
          RTTLMG      |  not enough geometry present on edge or
                      |  adjacent faces to recompute edge geometry
                      |
          RTTLRF      |  recomputation of edge geometry failed

  RETLEN attempts to replace geometry and tolerance as follows:

  . if all adjacent faces of the edge have surface geometry attached, a
    3-space curve of intersection will be derived by re-intersection of
    these surfaces. Extensive pre-checks will be made to ensure that
    tangent or near-tangent configurations (such as boundaries of
    Variable Radius Blends) are not attempted in this way. In these cases
    the token RTTLNT will be returned, and the edge geometry and tolerance
    will be unchanged. There will also be cases where the edge is not a near
    tangency, but nevertheless a new curve of intersection could not be
    computed (for example, the adjacent surfaces may simply not intersect to
    Parasolid default resolution); in such cases RTTLRF will be returned.
    Otherwise, if RETLEN is successful, 'retcod' will be set to RTTLOK.

    The particular simple case where one of the SP-curves lies within
    Parasolid default tolerance of the adjacent surface will be dealt
    with by either simplifying this SP-curve to its 3-space equivalent,
    of failing this, by transferring the SP-curve to the edge.

  . if not all face geometries are present, as at the boundary of a
    sheet body, the SP-curve present on the relevant fin will be first
    simplified to its 3-space equivalent (if it was a constant parameter
    curve) and this new curve attached to the edge. If no simplification
    was possible, the SP-curve itself will be transferred to the edge.
    In either case RTTLOK will be returned in 'retcod' if the geometry was
    successfully recomputed.

  If insufficient geometry was present to successfully replace the tolerance
  of the edge, RTTLMG (missing geometry) will be returned.

  RETLEN will also attempt to set the tolerance of each vertex of the edge
  to Parasolid default. It will only be able to do this if all the other
  edges meeting at the vertex in question have Parasolid default tolerance
  associated with them, and the new 3-space curve intersects at a unique
  point with all the other edge curves meeting there. Note that if both
  vertex tolerances are reset, the curve attached to the edge will no
  longer be a trimmed curve (type TYCUTR).