theorem Th19:
  ('not' p).x = 'not'(p.x)
proof
  set 9p = 'not' p;
A1: 9p is negative by QC_LANG1:def 19;
  then the_argument_of 9p = p by QC_LANG1:def 24;
  hence thesis by A1,Th18;
end;
