let H be Element of QC-WFF ; :: 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 QC_LANG1:def 22;
hence Subformulae H = (Subformulae (the_argument_of H)) \/ {H} by Th110; :: thesis: verum