for X being set
for f1, f2 being FinSequence ex Y being Subset of NAT st
( Seq ((f1 ^ f2) | X) = (Seq (f1 | X)) ^ (Seq (f2 | Y)) & ( for n being Nat st n > 0 holds
( n in Y iff n + (len f1) in X /\ (dom (f1 ^ f2)) ) ) )