\begin{cprog}
typedef struct QuadTree {
  double Data;
  struct QuadTree 
     *Children[4];
} QT;

void Sum3rdChildren(QT *T,
           double *Result) {
  double Ret;
  if (T == 0) { Ret = 0;
  } else {
    QT *Child3 =
      T[0].Children[3];
    double V;
    Sum3rdChildren(Child3, 
                   &V);
    Ret = V + T[0].Data;
  }
  *Result = Ret;
}
\end{cprog}