CRPLSU ( posn, normal, plane, ifail )
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  Create a planar surface.

Receives:
  KI_vec_position           posn           --- position on plane
  KI_vec_normal             normal         --- normal direction

Returns:
  KI_tag_surface           *plane          --- planar surface
  KI_cod_error             *ifail          --- failure code


Description:
  An unbounded plane is created passing through 'posn'. The surface normal
  is in the same direction as the given normal.