theorem
  H is disjunctive implies (M,v |= H iff M,v |= the_left_argument_of H
  or M,v |= the_right_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;
  hence thesis by ZF_MODEL:17;
end;
