let H, F be Element of QC-WFF ; :: thesis: ( H is_immediate_constituent_of F implies H is_proper_subformula_of F )
assume A1: H is_immediate_constituent_of F ; :: thesis: H is_proper_subformula_of F
hence H is_subformula_of F by Th72; :: according to QC_LANG2:def 21 :: thesis: H <> F
assume H = F ; :: thesis: contradiction
then len (@ H) = len (@ F) ;
hence contradiction by A1, Th71; :: thesis: verum