theorem
  H is negative implies the_argument_of (H/(x,y)) = (the_argument_of H)/
  (x, y )
proof
  assume
A1: H is negative;
  then H/(x,y) is negative by Th168;
  then
A2: H/(x,y) = 'not' the_argument_of (H/(x,y)) by ZF_LANG:def 30;
  H = 'not' the_argument_of H by A1,ZF_LANG:def 30;
  hence thesis by A2,Th156;
end;
