int cat; /* category of this block */
int pos; /* block number of this block */
int next; /* number of the next block */
int diffs; /* either block or offset (in head) of start of diffs */
int cat; /* category of this block */
int pos; /* block number of this block */
int next; /* number of the next block */
int diffs; /* either block or offset (in head) of start of diffs */