theorem Th14:
  H is_immediate_constituent_of 'X' F iff H = F
proof
  thus H is_immediate_constituent_of 'X' F implies H = F
  proof
A1: now
      given H1 such that
A2:   'X' F = H '&' H1 or 'X' F = H1 '&' H or 'X' F = H 'or' H1 or 'X'
F = H1 'or' H or 'X' F = H 'U' H1 or 'X' F = H1 'U' H or 'X' F = H 'R' H1 or
      'X' F = H1 'R' H;
      ('X' F).1 = 3 by Th12;
      hence contradiction by A2,Th12;
    end;
A3: now
      assume
A4:   'X' F = 'not' H;
      ('X' F).1 = 3 by Th12;
      hence contradiction by A4,Th12;
    end;
    assume H is_immediate_constituent_of 'X' F;
    then
    'X' F = 'not' H or 'X' F = 'X' H or ex H1 st 'X' F = H '&' H1 or 'X' F
= H1 '&' H or 'X' F = H 'or' H1 or 'X' F = H1 'or' H or 'X' F = H 'U' H1 or 'X'
    F = H1 'U' H or 'X' F = H 'R' H1 or 'X' F = H1 'R' H;
    hence thesis by A3,A1,FINSEQ_1:33;
  end;
  thus thesis;
end;
