meth isBalanced($)
   local NumLeave A L R X Y in
      fun{NumLeave T}
     local A B in
        if {T getLeft($)} == leaf then A = 1
        else A = {NumLeave {T getLeft($)}}
        end
        if {T getRight($)} == leaf then B = 1
        else B = {NumLeave {T getRight($)}}
        end
        A + B
     end
      end

      if @right == leaf then R = true X = 1
      else R = {@right isBalanced($)} X = {NumLeave @right}
      end

      if @left == leaf then L = true Y = 1
      else L = {@left isBalanced($)} Y = {NumLeave @left}
      end


      A = X - Y
      if A < 2 andthen ~2 < A andthen R andthen L  then true
      else false
      end
   end
end