reserve L for Boolean non empty RelStr;
reserve a,b,c,d for Element of L;

theorem
  'not' (Top L) = Bottom L
proof
  Bottom L "/\" Top L = Bottom L & Bottom L "\/" Top L = Top L by WAYBEL_1:3 ;
  then Bottom L is_a_complement_of Top L by WAYBEL_1:def 23;
  hence thesis by Th11;
end;
