procedure checkrots( var t : tree );
{*** check need for rotations ***}
var wl, wll, wr, wrr : integer;
begin
if t <> nil then with t^ do begin
    wl := wt( left );
    wr := wt( right );
    if wr > wl then begin
        {*** left rotation needed ***}
        wrr := wt( right^.right );
        if (wrr > wl) and (2*wrr >= wr) then
            begin lrot( t ); checkrots( left ) end
        else if wr-wrr > wl then begin
            rrot( right );  lrot( t );
            checkrots( left );  checkrots( right )
            end
        end
    else if wl > wr then begin
        {*** right rotation needed ***}
        wll := wt( left^.left );
        if (wll > wr) and (2*wll >= wl) then
            begin rrot( t ); checkrots( right ) end
        else if wl-wll > wr then begin
            lrot( left );   rrot( t );
            checkrots( left );  checkrots( right )
            end
        end
    end
end;