reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem
  X /\ (Y /\ Z) = (X /\ Y) /\ (X /\ Z)
proof
  thus X /\ (Y /\ Z) = X /\ X /\ Y /\ Z by Th16
    .= X /\ (X /\ Y) /\ Z by Th16
    .= (X /\ Y) /\ (X /\ Z) by Th16;
end;
