let G be V21() X_equal-in-line Y_equal-in-column Matrix of (TOP-REAL 2); :: thesis: cell G,0 ,(width G) = { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) }
A1: cell G,0 ,(width G) = (v_strip G,0 ) /\ (h_strip G,(width G)) by GOBOARD5:def 3;
A2: h_strip G,(width G) = { |[r,s]| where r, s is Real : (G * 1,(width G)) `2 <= s } by Th22;
A3: v_strip G,0 = { |[r,s]| where r, s is Real : r <= (G * 1,1) `1 } by Th18;
thus cell G,0 ,(width G) c= { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) } :: according to XBOOLE_0:def 10 :: thesis: { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) } c= cell G,0 ,(width G)

proof

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in cell G,0 ,(width G) or x in { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) } )
assume A4: x in cell G,0 ,(width G) ; :: thesis: x in { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) }
then x in v_strip G,0 by A1, XBOOLE_0:def 4;
then consider r1, s1 being Real such that
A5: x = |[r1,s1]| and
A6: r1 <= (G * 1,1) `1 by A3;
x in h_strip G,(width G) by A1, A4, XBOOLE_0:def 4;
then consider r2, s2 being Real such that
A7: x = |[r2,s2]| and
A8: (G * 1,(width G)) `2 <= s2 by A2;
s1 = s2 by A5, A7, SPPOL_2:1;
hence x in { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) } 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 : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) } or x in cell G,0 ,(width G) )
assume x in { |[r,s]| where r, s is Real : ( r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) } ; :: thesis: x in cell G,0 ,(width G)
then A9: ex r, s being Real st
( x = |[r,s]| & r <= (G * 1,1) `1 & (G * 1,(width G)) `2 <= s ) ;
then A10: x in h_strip G,(width G) by A2;
x in v_strip G,0 by A3, A9;
hence x in cell G,0 ,(width G) by A1, A10, XBOOLE_0:def 4; :: thesis: verum