for k being Nat
for D being set
for f1, f2 being FinSequence of D
for G being Matrix of D st 1 < k & k + 1 <= len f1 & k + 1 <= len f2 & f1 is_sequence_on G & f1 | k = f2 | k & f1 turns_left k -' 1,G & f2 turns_left k -' 1,G holds
f1 | (k + 1) = f2 | (k + 1)