list sort( r )
    list r;

    {
    list head[M], tail[M];
    int i, j, h;
    for (i=D; i>0; i--) {
        for (j=0; j<M; j++) head[j] = NULL;
        while ( r != NULL ) {
            h = charac( i, r->k );
            if ( head[h]==NULL ) head[h] = r;
            else    tail[h]->next = r;
            tail[h] = r;
            r = r->next;
            };
        /*** Concatenate lists ***/
        r = NULL;
        for (j=M-1; j>=0; j--)
            if ( head[j] != NULL ) {
                tail[j]->next = r;
                r = head[j];
                }
        };
    return( r );
    };