datatype
         'label evenTree = Empty |
             Enode of 'label * 'label oddTree * 'label oddTree
     and
         'label oddTree =
             Onode of 'label * 'label evenTree * 'label evenTree;

     val t1 = Onode(1,Empty,Empty);

     val t2 = Onode(2,Empty,Empty);

     val t3 = Enode(3,t1,t2);

     val t4 = Onode(4,t3,Empty);

     val t5 = Enode(5,t4,t4);