procedure P(S)
    begin
    P(active)
    if(S.val >0)
    begin
      S.val=S.val-1
      V(active)
    end
    else
    begin
      P(S_count)
      count++
      V(S_count)
      V(active)
      Pb(S)
      P(S_count)
      count--
      V(S_count)
    end
    end;

    procedure V(S)
    begin
    P(active)
    P(S_count)
    if(count>0)
    begin
      V(S_count)
      Vb(S)
      V(active)
    end
    else
    begin
      V(S_count)
      S.val=S.val+1
      V(active)
    end
    end