Apropos of the virtue of negative subscripts.

> by pointing into the middle of another data structure you've created a data aliasing situation

Not if all references are relative to the offset pointer. The following example is silly,  but I recently used exactly this trick to simplify calculations on a first-quadrant (x,y) grid by supplying "out-of-bounds" zeroes at (x,-2), (x,-1) and (-1,y). It was much cleaner to access the zeroes like ordinary elements than to provide special code to handle the boundary conditions.

/* Fill an N-element array with Fibonacci numbers, f(i), where 0<=i<N. 

   The recursion accesses a zero "out of bounds" at i=-1 */

const int N = 100;

int base[N+1];

#define fib(i) base[(i)+1]

void fill() {

    int i;

    fib(0) = 1;

    for(i=1; i<N; i++)

        fib(i) = fib(i-2) + fib(i-1);

}

Doug