theorem Th26:
  A,B are_independent_respect_to P implies ([#] Sigma \ A),([#]
  Sigma \ B) are_independent_respect_to P
proof
  assume A,B are_independent_respect_to P;
  then A,([#] Sigma \ B) are_independent_respect_to P by Th25;
  then ([#] Sigma \ B),A are_independent_respect_to P;
  then ([#] Sigma \ B),([#] Sigma \ A) are_independent_respect_to P by Th25;
  hence thesis;
end;
