let T be Subset of MC-wff; :: thesis: ( T is S4_theory iff CnS4 T = T )
hereby :: thesis: ( CnS4 T = T implies T is S4_theory )
assume A1: T is S4_theory ; :: thesis: CnS4 T = T
A2: CnS4 T c= T by A1, Def23;
T c= CnS4 T by Th89;
hence CnS4 T = T by A2, XBOOLE_0:def 10; :: thesis: verum
end;
thus ( CnS4 T = T implies T is S4_theory ) ; :: thesis: verum