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;
