CRRVSU ( profil, point, direct, nopts, opts, optdta, revsur, ifail )
====================================================================

  Create a surface of revolution

Receives:
  KI_tag_curve             *profil         --- curve to revolve
  KI_vec_position           point          --- point on revolution axis
  KI_vec_axis               direct         --- direction of revolution axis
 <KI_int_nitems>           *nopts          --- number of options supplied
  KI_cod_crop               opts[nopts]    --- array of options
 <KI_tag_list>              optdta[nopts]  --- array of option data

Returns:
  KI_tag_surface           *revsur         --- resulting surface of revolution
  KI_cod_error             *ifail          --- error code

Specific errors:
  KI_impossible_swing       Cannot determine spun geometry
  KI_su_self_intersect      Surface would be self-intersecting
  KI_unsuitable_entity      Not one of the allowed curve types
  KI_wrong_direction        Parameters in wrong order
  KI_invalid_geometry       Invalid curve
  KI_bad_parameter          End parameter out of range
                            Start parameter out of range
  KI_bad_parametric_prop    inappropriate property
  KI_bad_option_data        Should be exactly 2 values
                            option data missing

Description:

  This function creates a spun surface or surface of revolution.

  The curve 'profil' is revolved about the axis defined by the point
  vector 'point' and the direction vector 'direct', the new surface
  being the envelope of the curve. The sense of the revolution appears
  clockwise when viewed along the direction of the axis.

  The curve must be one of the following:

  line
  circle
  ellipse
  B-curve

  The options allow the caller to indicate whether simplification is to
  be attempted and to specify a parameter range which indicates which
  part of the curve to use. The possible entries are:


      token   |           meaning                 |         data
      ---------------------------------------------------------------------
              |                                   |
      CROPSI  |      Simplify to equivalent       |         none
              |         analytic surface          |
              |                                   |
      CROPPR  |    Parameter range on required    |  list of two doubles
              |        section of 'profil'        |
              |                                   |


  The simplification token is CROPSI. If this is not given then the
  function will always return a surface of type TYSUSU. If it is given

  then, if possible, the function will return an simpler surface which
  is equivalent to the surface of revolution. Normally the simpler
  surface is analytic; for example, a spun line which intersects the
  axis is equivalent to a cone. Sometimes the simpler surface is a spun
  surface with a profile different to 'profil'. This can occur when the
  whole of 'profil' would sweep the spun surface twice, i.e. the surface
  would be self intersecting at all its points. For example, this occurs
  when 'profil' is a circle which lies in a plane perpendicular to the
  plane containing its centre and the spin axis. In this case, the
  profile of the resulting surface is a B-curve equivalent to the part
  of 'profil' specified using CROPPR (although the parameterisation of
  the new profile will, in general, be different to that of 'profil').


  If the token CROPPR is supplied, then a list of two doubles must also
  be supplied as data. These two doubles specify a parameter range
  indicating the part of the curve to be spun. A parameter range may be
  specified for any case but will only be important in cases where the
  curve intersects the axis. In these cases, it is ambiguous as to which
  part of the curve should be spun and this option provides a way of
  choosing one part to be spun. The parameters must be given in
  ascending order. The curve is extended from the given parameter range
  until it meets the spin axis, and this becomes the portion of curve to
  be revolved. If ambiguous data is supplied and SLIPSI (see SEINTP) is
  non-zero, then an error is signalled, otherwise (if SLIPSI is zero)
  then the part of the curve used is undefined.


  A self intersecting surface may be returned, but it is not possible to
  spin a line about an axis equivalent to the line itself.

  If 'profil' is not an orphan, the resulting surface will be created as
  construction geometry in the owning part. A curve which is a dependent of
  another entity may be revolved - for example, a single curve may be the
  underlying curve of two spun surfaces.