theorem :: FINSEQ_6:190

for D being non empty set
for f1 being FinSequence of D
for i1, i2, j being Nat st 1 <= i2 & i2 <= i1 & i1 <= len f1 & 1 <= j & j <= (i1 -' i2) + 1 holds
(mid (f1,i1,i2)) . j = f1 . ((i1 -' j) + 1)