procedure checkrots( var t : tree );
var wbal : real;
begin
with t^ do begin
    wbal := wt(left) / weight;
    if wbal > 0.707011 then
        {*** left subtree too heavy: right rotation needed ***}
        if wt(left^.left) / wt(left) > 0.414213 then    rrot( t )
            else    begin   lrot( left ); rrot( t ) end
    else    if wbal < 0.292893 then
        {*** right subtree too heavy: left rotation needed ***}
        if wt(right^.left) / wt(right) < 0.585786 then  lrot( t )
            else    begin   rrot( right ); lrot( t ) end
    end
end;