
theorem Th6:
  for L being Dneg non empty OrthoRelStr, x being Element of L holds x`` = x
proof
  let L be Dneg non empty OrthoRelStr, x be Element of L;
  consider f being Function of L,L such that
A1: f = the Compl of L and
A2: f is involutive by OPOSET_1:def 3;
  f.x = x` & f.(f.x) = x by A1,A2,PARTIT_2:def 3,ROBBINS1:def 3;
  hence thesis by A1,ROBBINS1:def 3;
end;
