TWSUFA ( faces, surfs, senses, state, ifail )
=============================================

  Tweak the surface(s) of face(s).

Receives:
  KI_tag_list_face         *faces          --- face(s) to tweak
  KI_tag_list_surface      *surfs          --- new surface(s) for face(s)
  KI_tag_list_int          *senses         --- senses for surfaces

Returns:
  KI_cod_rtlo              *state          --- state of the body
                                           ---   RTLOOK => Valid
                                           ---   RTLONG => Negated
                                           ---   RTLOSX => Self-Intersecting
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_cant_do_tweak          Tweak cannot be performed
                            Tweak cannot be applied to face on boundary
                            Tweak cannot be applied to a rubber face
  KI_unsuitable_entity      Mixed body
                            Surface is too complex
  KI_general_body           General bodies not supported
  KI_bad_sharing            attempt to illegally share a surface
  KI_invalid_geometry       Surface fails checks
  KI_not_same_length        Lists not of equal length
  KI_duplicate_list_item    Face appears twice in argument list
  KI_not_in_same_part       All faces are not from the same part

Description:
  The surface geometries of the faces are replaced and the curves and points of
  edges and vertices are recalculated.

  Where edges or faces (sharing a vertex or edge at the boundary of the faces to
  be tweaked) have geometry which is inextendable, it may not be possible to
  recalculate vertices or edges. This is because the curve or surface does not
  intersect the new surface. With such edges which have a user defined tolerance
  this is likely to occur since SP-curves are inextendable.

  If the faces are on a sheet body, they must not share an edge with any rubber
  faces.

  The entries in 'senses' define whether the corresponding surface is to
  be reversed before being attached to a face. After the tweak operation
  has been performed all the entities in 'faces' will have their senses
  set to true.

  The number of surfaces and the number of senses must be identical. The number
  of surfaces supplied must either be one, in which case every face is tweaked
  to the same surface, or equal to the number of faces, in which case every face
  is tweaked to the corresponding surface.

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

  In general this procedure may give rise to self-intersecting object
  boundaries which could cause unpredictable errors later.

  If local checking is on, (see SEINTP and OUINTP) consistency checks will be
  made on newly created topological and geometrical entities, and the state
  of the body returned. A state of RTLOOK indicates the body is valid. A state
  of RTLONG indicates that the result body was originally "inside out" but has
  been negated, and is now "positive" (has positive volume) and valid. A state
  of RTLOSX indicates the body is self-intersecting and further modelling
  operations on it may fail. It is the responsibility of the calling routine to
  make any necessary reparation.

  If the interface parameter for local checking is switched off, the state
  returned will be RTLOOK regardless.

  A self-intersecting body can be returned even if the ifail is zero.

  This function is not supported for general bodies.