\ B-tree of B-trees                                                  vandys
\ Key returns a B-tree; an empty B-tree is created on first reference
only extensions definitions

Btree -> subclass: BtBtree
BtBtree -> :method @ { key self -- bt }
   key self super-> @ if   exit   then
   Btree -> new { bt }   bt key self super-> !   bt method;
BtBtree -> :method ! ( val key self -- )
   1 abort" BtBtree:! not implemented" method;
BtBtree -> :method .elems ( self -- )   0   ['] (.leaf)   rot -> do method;
BtBtree -> :method in? ( key self -- ? )
   super-> @   dup if nip then method;