   code demonstrates its use for the scenario where 
    \(s_{k+1}\) is negative, 
    \(s_{k+2}\) is negative, 
    \(A0\) is zero, and
    \(A3\) is zero.  The expression that is generated is the
    equation for \(C_x(t)\) in that scenario (the <code>px</code>
    terms in the expression are the x-values from the four
    control points that are controlling the curve, e.g.,
    <code>px0</code> corresponds to the x-component of \(P_k\);
    <code>s1</code>  and <code>s2</code> correspond to 
    \(s_{k+1