untyped hierarchy block connector.

   derive block cube horizontal-bar ground.
   derive2 connector2 column.
   derive3 connector3 arch.
   derive1 connector1 left right.

insert
   (arch [horizontal-bar:*] [cube:*cl] [cube:*cr])
   (left ?cl)
   (right ?cr)
   (column ?cl [ground:*])
   (column ?cr [ground:*]).

select
   (arch [horizontal-bar:*] [cube:*cl] [cube:*cr])
   (left ?cl)
   (right ?cr)
   (column ?cl [ground:*])
   (column ?cr [ground:*]).