theorem Th27: :: CHAIN_1:30

for d being non zero Nat
for l, r, l9, r9 being Element of REAL d st ( for i being Element of Seg d holds l . i <= r . i ) & ( for i being Element of Seg d holds r9 . i < l9 . i ) holds
( cell (l,r) c= cell (l9,r9) iff ex i being Element of Seg d st
( r . i <= r9 . i or l9 . i <= l . i ) )