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 );
	};