theorem
  A c= B? implies A? c= B?
proof
  <%>E in B? by Th78;
  then
A1: {<%>E} c= B? by ZFMISC_1:31;
  assume A c= B?;
  then {<%>E} \/ A c= B? by A1,XBOOLE_1:8;
  hence thesis by Th76;
end;
