def plane
      # Such a change is only possible if the Slice instance has a Contour with at least three Coordinates:
      raise "This Slice does not contain a Contour. Plane determination is not possible." if @contours.length == 0
      raise "This Slice does not contain a Contour with at least 3 Coordinates. Plane determination is not possible." if @contours.first.coordinates.length < 3
      # Get three coordinates from our Contour:
      contour = @contours.first
      num_coords = contour.coordinates.length
      c1 = contour.coordinates.first
      c2 = contour.coordinates[num_coords / 3]
      c3 = contour.coordinates[2 * num_coords / 3]
      return Plane.calculate(c1, c2, c3)
    end
def plane
      # Such a change is only possible if the Slice instance has a Contour with at least three Coordinates:
      raise "This Slice does not contain a Contour. Plane determination is not possible." if @contours.length == 0
      raise "This Slice does not contain a Contour with at least 3 Coordinates. Plane determination is not possible." if @contours.first.coordinates.length < 3
      # Get three coordinates from our Contour:
      contour = @contours.first
      num_coords = contour.coordinates.length
      c1 = contour.coordinates.first
      c2 = contour.coordinates[num_coords / 3]
      c3 = contour.coordinates[2 * num_coords / 3]
      return Plane.calculate(c1, c2, c3)
    end
Gives a Plane corresponding to this Slice geometry. The plane is
 calculated from coordinates belonging to this instance.

 @return [Plane] the derived plane
 @raise [RuntimeError] unless the required number of coordinates are present (at least 3)