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;

theorem Th50:
  not H is_immediate_constituent_of x '=' y
proof
  assume H is_immediate_constituent_of x '=' y;
  then
A1: x '=' y = 'not' H or ( ex H1 st x '=' y = H '&' H1 or x '=' y = H1 '&' H
  ) or ex z st x '=' y = All(z,H);
  (x '=' y).1 = 0 by Th15;
  hence contradiction by A1,Th16,Th17,FINSEQ_1:41;
end;
