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
  H is disjunctive implies the_left_argument_of H = the_argument_of
  the_left_argument_of the_argument_of H
proof
  assume H is disjunctive;
  then H = (the_left_argument_of H) 'or' (the_right_argument_of H) by
ZF_LANG:41;
  then the_argument_of H = 'not'(the_left_argument_of H) '&' 'not' (
  the_right_argument_of H) by Th3;
  then the_left_argument_of the_argument_of H = 'not'(the_left_argument_of H)
  by Th4;
  hence thesis by Th3;
end;
