let i, j be Nat; :: thesis: for G being Go-board st 1 <= i & i < len G & 1 <= j & j < width G holds
Int (cell (G,i,j)) = { |[r,s]| where r, s is Real : ( (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) }
let G be Go-board; :: thesis: ( 1 <= i & i < len G & 1 <= j & j < width G implies Int (cell (G,i,j)) = { |[r,s]| where r, s is Real : ( (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) } )
assume that
A1: ( 1 <= i & i < len G ) and
A2: ( 1 <= j & j < width G ) ; :: thesis: Int (cell (G,i,j)) = { |[r,s]| where r, s is Real : ( (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) }
A3: Int (h_strip (G,j)) = { |[r,s]| where r, s is Real : ( (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) } by A2, Th17;
cell (G,i,j) = (v_strip (G,i)) /\ (h_strip (G,j)) by GOBOARD5:def 3;
then A4: Int (cell (G,i,j)) = (Int (v_strip (G,i))) /\ (Int (h_strip (G,j))) by TOPS_1:17;
A5: Int (v_strip (G,i)) = { |[r,s]| where r, s is Real : ( (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 ) } by A1, Th14;
thus Int (cell (G,i,j)) c= { |[r,s]| where r, s is Real : ( (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 & (G * (1,j)) `2 < s & s < (G * (1,(j + 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 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) } c= Int (cell (G,i,j))

proof

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

end;

let x be object ; :: 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 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) } or x in Int (cell (G,i,j)) )
assume x in { |[r,s]| where r, s is Real : ( (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) } ; :: thesis: x in Int (cell (G,i,j))
then A11: ex r, s being Real st
( x = |[r,s]| & (G * (i,1)) `1 < r & r < (G * ((i + 1),1)) `1 & (G * (1,j)) `2 < s & s < (G * (1,(j + 1))) `2 ) ;
then A12: x in Int (h_strip (G,j)) by A3;
x in Int (v_strip (G,i)) by A5, A11;
hence x in Int (cell (G,i,j)) by A4, A12, XBOOLE_0:def 4; :: thesis: verum