theorem :: FINSEQ_3:85

for p being FinSequence
for A being set
for n being Nat st n in dom p holds
for B being finite set holds
( not B = { k where k is Element of NAT : ( k in dom p & k <= n & p . k in A ) } or p . n in A or (p - A) . (n - (card B)) = p . n )