CellHdr
#define MIN_CELL_SIZE (sizeof(CellHdr)+4)
#define MX_LOCAL_PAYLOAD ((USABLE_SPACE/4-(sizeof(CellHdr)+sizeof(Pgno)))&~3)
CellHdr h; /* The cell header */
n += sizeof(CellHdr);
assert( sizeof(CellHdr)==12 );
typedef struct CellHdr CellHdr;