theorem Th56:
  ('not' p).(x,y) = 'not' (p.(x,y))
proof
  set S = [p,Sbst(x,y)];
  S`1 = p & S`2 = Sbst(x,y);
  then ('not' p).(x,y) = CQC_Sub(['not' p,Sbst(x,y)]) & Sub_not S = ['not' p,
  Sbst(x,y)] by SUBSTUT1:def 20,SUBSTUT2:def 1;
  then ('not' p).(x,y) = 'not' CQC_Sub(S) by SUBSTUT1:29;
  hence thesis by SUBSTUT2:def 1;
end;
