CRTRCU ( basis_curve, parm_1, parm_2, trimmed_curve, ifail )
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  Creates a trimmed curve

Receives:
  KI_tag_curve             *basis_curve    --- underlying basis curve
  KI_dbl_parameter         *parm_1         --- start parameter
  KI_dbl_parameter         *parm_2         --- end parameter

Returns:
  KI_tag_curve             *trimmed_curve   --- trimmed basis curve
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_curve_too_short        trimmed curve is shorter than linear resolution
  KI_bad_parameter          parameter 'parm_2' is out of range
                            parameter 'parm_1' is out of range
                            parm_2 is less than parm_1 on non-periodic
  KI_unsuitable_entity      trimmed polylines not allowed
                            b curve is referenced

Description:
  Creates a trimmed curve from a curve and two bound parameters defining points
  on the curve.

  The arguments are:

  The curve 'basis_curve' of the trimmed curve:
  . If the basis curve is already owned by a part, then the trimmed curve will
    also be owned by that part.
  . If the curve supplied is already a trimmed curve then its start and end
    points will be changed to be 'parm_1' and 'parm_2' repectively. This allows
    re-trimming of trimmed curves. The trimmed curve returned will have the same
    tag as that supplied.

  Parameter 'parm_1':
  . The parameter of the start point of the trimmed curve.

  Parameter 'parm_2':
  . The parameter of the end point of the trimmed curve.

  If the basis curve is not periodic, the parameters must be valid for the curve
  (i.e. in the range returned by ENCUPA) and the corresponding points must be
  more than linear resolution apart. Furthermore 'parm_1' must be less than
  'parm_2'.

  If the basis curve is periodic, 'parm_1' and 'parm_2' can be in any range.
  If the points corresponding to 'parm_1' and 'parm_2' are less than linear
  resolution apart, then a trimmed curve is made from the whole basis curve.

  If the basis curve is closed but not periodic, then a trimmed curve is made
  from the whole basis curve if the points corresponding to parm_1 and parm_2
  are within linear resolution of the ends of the curve and within linear
  resolution of each other.