for k, k9 being Nat
for d being non zero Nat
for l, r, l9, r9 being Element of REAL d
for G being Grating of d st k < k9 & k9 <= d & cell (l,r) in cells (k,G) & cell (l9,r9) in cells (k9,G) & cell (l,r) c= cell (l9,r9) holds
ex i being Element of Seg d st
( ( l . i = l9 . i & r . i = l9 . i ) or ( l . i = r9 . i & r . i = r9 . i ) )