let G be Go-board; :: thesis: Int (cell G,0 ,0 ) = { |[r,s]| where r, s is Real : ( r < (G * 1,1) `1 & s < (G * 1,1) `2 ) }
cell G,0 ,0 = (v_strip G,0 ) /\ (h_strip G,0 ) by GOBOARD5:def 3;
then A1: Int (cell G,0 ,0 ) = (Int (v_strip G,0 )) /\ (Int (h_strip G,0 )) by TOPS_1:46;
A2: Int (v_strip G,0 ) = { |[r,s]| where r, s is Real : r < (G * 1,1) `1 } by Th15;
A3: Int (h_strip G,0 ) = { |[r,s]| where r, s is Real : s < (G * 1,1) `2 } by Th18;
thus Int (cell G,0 ,0 ) c= { |[r,s]| where r, s is Real : ( r < (G * 1,1) `1 & s < (G * 1,1) `2 ) } :: according to XBOOLE_0:def 10 :: thesis: { |[r,s]| where r, s is Real : ( r < (G * 1,1) `1 & s < (G * 1,1) `2 ) } c= Int (cell G,0 ,0 )

proof

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