function select( s : integer; var r : RecordArray;
			 lo, up : integer ) : typekey;

	var	i, j : integer;
		tempr : ArrayEntry;
	begin
	s := s+lo-1;
	if (s<lo) or (s>up) then Error {*** selection out of bounds ***}
	else begin
		while (up>=s) and (s>=lo) do begin
			i := lo;
			j := up;
			tempr := r[s];   r[s] := r[lo];   r[lo] := tempr;
			{*** split file in two ***}
			while i<j do begin
				while r[j].k > tempr.k do
					j := j-1;
				r[i] := r[j];
				while (i<j) and (r[i].k<=tempr.k) do
					i := i+1;
				r[j] := r[i]
				end;
			r[i] := tempr;
			{*** select subfile ***}
			if s<i then up := i-1
				else lo := i+1
			end;
		select := r[s].k
		end
	end;