let i be Element of NAT ; :: thesis: for G being V21() X_equal-in-line Y_equal-in-column Matrix of (TOP-REAL 2) st 1 <= i & i < len G holds
cell G,i,0 = { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) }
let G be V21() X_equal-in-line Y_equal-in-column Matrix of (TOP-REAL 2); :: thesis: ( 1 <= i & i < len G implies cell G,i,0 = { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } )
A1: cell G,i,0 = (v_strip G,i) /\ (h_strip G,0 ) by GOBOARD5:def 3;
assume ( 1 <= i & i < len G ) ; :: thesis: cell G,i,0 = { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) }
then A2: v_strip G,i = { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 ) } by Th20;
A3: h_strip G,0 = { |[r,s]| where r, s is Real : s <= (G * 1,1) `2 } by Th21;
thus cell G,i,0 c= { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } :: according to XBOOLE_0:def 10 :: thesis: { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } c= cell G,i,0

proof

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in cell G,i,0 or x in { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } )
assume A4: x in cell G,i,0 ; :: thesis: x in { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) }
then x in v_strip G,i by A1, XBOOLE_0:def 4;
then consider r1, s1 being Real such that
A5: x = |[r1,s1]| and
A6: ( (G * i,1) `1 <= r1 & r1 <= (G * (i + 1),1) `1 ) by A2;
x in h_strip G,0 by A1, A4, XBOOLE_0:def 4;
then consider r2, s2 being Real such that
A7: x = |[r2,s2]| and
A8: s2 <= (G * 1,1) `2 by A3;
s1 = s2 by A5, A7, SPPOL_2:1;
hence x in { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } by A5, A6, A8; :: thesis: verum

end;

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } or x in cell G,i,0 )
assume x in { |[r,s]| where r, s is Real : ( (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) } ; :: thesis: x in cell G,i,0
then A9: ex r, s being Real st
( x = |[r,s]| & (G * i,1) `1 <= r & r <= (G * (i + 1),1) `1 & s <= (G * 1,1) `2 ) ;
then A10: x in h_strip G,0 by A3;
x in v_strip G,i by A2, A9;
hence x in cell G,i,0 by A1, A10, XBOOLE_0:def 4; :: thesis: verum