let A be QC-alphabet ; :: thesis: for H being Element of QC-WFF A st H is negative holds
Subformulae H = (Subformulae (the_argument_of H)) \/ {H}
let H be Element of QC-WFF A; :: 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 24;
hence Subformulae H = (Subformulae (the_argument_of H)) \/ {H} by Th88; :: thesis: verum