function getDerivative(derivative, t, vs) {
  // the derivative of any 't'-less function is zero.
  var n = vs.length - 1,
      _vs,
      value,
      k;
  if (n === 0) {
    return 0;
  }

  // direct values? compute!
  if (derivative === 0) {
    value = 0;
    for (k = 0; k <= n; k++) {
      value += binomials(n, k) * Math.pow(1 - t, n - k) * Math.pow(t, k) * vs[k];
    }
    return value;
  } else {
    // Still some derivative? go down one order, then try
    // for the lower order curve's.
    _vs = new Array(n);
    for (k = 0; k < n; k++) {
      _vs[k] = n * (vs[k + 1] - vs[k]);
    }
    return getDerivative(derivative - 1, t, _vs);
  }
}
Compute the curve derivative (hodograph) at t.