theorem Th29: :: PRE_POLY:30

for D being set
for F being FinSequence of D *
for i, j being Element of NAT st i in dom F & j in dom (F . i) holds
( (Sum (Card (F | (i -' 1)))) + j in dom (FlattenSeq F) & (F . i) . j = (FlattenSeq F) . ((Sum (Card (F | (i -' 1)))) + j) )