ENEXEN ( entity, dir1, dir2, dir3, posn, ext, ifail )
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  Enquire extreme point of entity

Receives:
  KI_tag_topology          *entity         --- face or edge
  KI_vec_direction          dir1           --- first direction
  KI_vec_direction          dir2           --- second direction
  KI_vec_direction          dir3           --- third direction

Returns:
  KI_vec_position           posn           --- position of extreme point
  KI_tag_topology          *ext            --- face, edge or vertex on
                                           --- which extreme lies
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_cant_find_extreme      failure
  KI_missing_geom           entity has missing geometry
  KI_wrong_entity           entity is not a face or an edge
  KI_coplanar               directions are coplanar

Description:
  The entity must be a face or an edge.

  The extreme point on the face or edge which is furthest in the direction
  'dir1' is returned, unless there is more than one such point, in which case
  'dir2' and 'dir3' are used successively to reduce the number of extreme
  points to one.

  If the extreme point lies on a vertex or an edge, this is returned in 'ext'.
  Otherwise the face is returned.

  The entity must not have missing geometry.

  The three directions must not be coplanar.