In this next stage, two further curves are added to the surface definition to refine its shape.
59. Create X-Topology Curve, starting by snapping halfway along the bottom curve.
60. Place a control point in the centre of the surface.
61. Finishing off by snapping to the top curve.
62. Create another X-Topology Curve, starting by snapping halfway up the left curve.
63. The next control point should snap to halfway up to the curve drawn just previously.
64. Finishing off by snapping a control point halfway up the right curve.
