theorem Th9:
  bound_in Ex(x,p) = x & the_scope_of Ex(x,p) = p
proof
  Ex(x,p) is existential;
  then Ex(x,p) = Ex(bound_in Ex(x,p), the_scope_of Ex(x,p)) by ZF_LANG:45;
  hence thesis by ZF_LANG:34;
end;
