</p>
    <p>
    It is easy to generate an expression for the unit tangent from 
    the X-spline
    tangent expression by dividing by the square-root of the sum of 
    squares of the x-component and y-component;
    the <code>xsplineUnitTangentExpr</code> 
    function can do this.  We can then repeat the process, 
    differentiating the unit tangent to get an expression for the normal 
    function and then 
    generating an expression for the unit normal.  This is provided by the
    <code>xsplineUni