formulas(u_shape).
  U   <->   on(a5,a6) & left(a6,a7) & left(a7,a8) &   on(a9,a8).
  Ur  <-> left(a6,a5) &   on(a6,a7) &   on(a7,a8) & left(a8,a9).
  Urr <->   on(a6,a5) & left(a7,a6) & left(a8,a7) &   on(a8,a9).
  Urrr<-> left(a5,a6) &   on(a7,a6) &   on(a8,a7) & left(a9,a8).
  U | Ur | Urr | Urrr.            
  U    -> -Ur & -Urr & -Urrr.     
  Ur   -> -U  & -Urr & -Urrr.
  Urr  -> -U  & -Ur  & -Urrr.
  Urrr -> -U  & -Ur  & -Urr. 
end_of_list.

