theorem Th6:
  m gcd n = 1 & k gcd n = 1 implies m*k gcd n = 1
proof
  assume (m gcd n)=1 & (k gcd n)=1;
  then m,n are_coprime & k,n are_coprime by INT_2:def 3;
  then m*k,n are_coprime by INT_2:26;
  hence thesis by INT_2:def 3;
end;
