reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;
reserve F,F1,G,G1,H,H1 for ZF-formula;
reserve sq,sq9 for FinSequence;
reserve L,L9 for FinSequence;
reserve j,j1 for Element of NAT;

theorem Th67:
  not F is_proper_subformula_of x '=' y
proof
  assume F is_proper_subformula_of x '=' y;
  then ex G st G is_immediate_constituent_of x '=' y by Th63;
  hence contradiction by Th50;
end;
