reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i for Integer;
reserve r for Real;
reserve p for Prime;

theorem
  (a <> 0 or b <> 0) & c <> 0 &
  a,b,c are_mutually_coprime implies a*b,c are_coprime by EULER_1:14;
