datatype 'label btree =
         Empty |
         Node of 'label * 'label btree * 'label btree;

     fun lower(nil) = nil
     |   lower(c::cs) = (Char.toLower c)::lower(cs);

     fun strLT(x,y) =
             implode(lower(explode x)) <
                 implode(lower(explode y));

     fun insert lt Empty x = Node(x,Empty,Empty)
     |   insert lt (T as Node(y,left,right)) x =
             if lt(x,y) then Node(y,(insert lt left x),right)
             else if lt(y,x) then Node(y,left,(insert lt right x))
             else (* x=y *) T; (* do nothing; x was
                                  already there *)

     val t = Node("ML",
                 Node("as",
                     Node("a",Empty,Empty),
                     Node("in",Empty,Empty)
                 ),
                 Node("types",Empty,Empty)
             );

     insert strLT t "function";