signature ELEMENT = sig
         type element;
         val similar : element * element -> bool;
     end;

     signature BTREE = sig
         structure Element: ELEMENT;
         eqtype elt;
         sharing type elt = Element.element;
         datatype btree = Empty | Node of elt * btree * btree;
         val leaf : elt -> btree;
         val build : elt * btree * btree -> btree;
         val lookup : elt * btree -> bool
     end;

     signature TREE = sig
         structure Element: ELEMENT;
         eqtype elt;
         sharing type elt = Element.element;
         datatype tree = Tree of elt * tree list;
         val build : elt * tree list -> tree;
         val lookup : elt * tree -> bool
     end;

     signature ALLTREES = sig
         structure Btree: BTREE;
         structure Tree: TREE;
         sharing Btree.Element = Tree.Element;
         sharing type Btree.elt = Tree.elt
     end;