Function backward

src/simplicity/eval.c:132–135

Given a read frame, move its cursor backwards by 'n' cells. * * Precondition: n <= frame->offset */

 * Precondition: n <= frame->offset
 */
static void backward(frameItem* frame, size_t n) {
  simplicity_debug_assert(n <= frame->offset);
  frame->offset -= n;
}

/* Given a write frame, advance its cursor by 'n' cells.
 *