theorem Th51:
  not H is_immediate_constituent_of x 'in' y
proof
  assume H is_immediate_constituent_of x 'in' y;
  then
A1: x 'in' y = 'not' H or ( ex H1 st x 'in' y = H '&' H1 or x 'in' y = H1
  '&' H ) or ex z st x 'in' y = All(z,H);
  (x 'in' y).1 = 1 by Th15;
  hence contradiction by A1,Th16,Th17,FINSEQ_1:41;
end;
