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    Watertight solids for gCAD3D

    gCAD3D does not provide watertight solids yet.
      All curves are polygonalized individually;
      the common boundary of two sufaces side-by-side is polygonalized twice;
      this can cause small slits between the boundaries.

    gCAD3D V2.35 changes:
    - all curve-modifications (eg CUT) create now trimmed-curves
      eg the result of trimming a circle is a (trimmed -) curve;
      eg "S20 = CUT C20 L20" // trimmed-curve S20 = cut circle C20 with line L20
    - all curves now have a polygonal-representation with BoundaryMarks
      the display- function uses the polygonal-representation;
      the functions for polygonal-representation are PRCV_*.

    TODO  V2.36:
    - modify surface-functions to use the polygonal-representation of its boundaries