let H be ZF-formula; :: thesis: ( H is negative implies the_argument_of H is_proper_subformula_of H )
assume H is negative ; :: thesis: the_argument_of H is_proper_subformula_of H
then the_argument_of H is_immediate_constituent_of H by Th56;
hence the_argument_of H is_proper_subformula_of H by Th61; :: thesis: verum