reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;

theorem Th3:
  the_argument_of 'not' p = p
proof
  'not' p is negative;
  hence thesis by ZF_LANG:def 30;
end;
