let H be ZF-formula; :: thesis: ( H is negative implies Subformulae H = (Subformulae (the_argument_of H)) \/ {H} )
assume H is negative ; :: thesis: Subformulae H = (Subformulae (the_argument_of H)) \/ {H}
then H = 'not' (the_argument_of H) by Def30;
hence Subformulae H = (Subformulae (the_argument_of H)) \/ {H} by Th82; :: thesis: verum