let i1, j1, j2 be Nat; :: thesis: for G1, G2 being Go-board st Values G1 c= Values G2 & [i1,j1] in Indices G1 & 1 <= j2 & j2 <= width G2 & G1 * (i1,j1) = G2 * (1,j2) holds
i1 = 1

let G1, G2 be Go-board; :: thesis: ( Values G1 c= Values G2 & [i1,j1] in Indices G1 & 1 <= j2 & j2 <= width G2 & G1 * (i1,j1) = G2 * (1,j2) implies i1 = 1 )
assume that
A1: Values G1 c= Values G2 and
A2: [i1,j1] in Indices G1 and
A3: ( 1 <= j2 & j2 <= width G2 ) and
A4: G1 * (i1,j1) = G2 * (1,j2) ; :: thesis: i1 = 1
set p = G1 * (1,j1);
A5: ( 1 <= j1 & j1 <= width G1 ) by ;
assume A6: i1 <> 1 ; :: thesis: contradiction
1 <= i1 by ;
then A7: 1 < i1 by ;
i1 <= len G1 by ;
then A8: (G1 * (1,j1)) `1 < (G1 * (i1,j1)) `1 by ;
0 <> len G1 by MATRIX_0:def 10;
then 1 <= len G1 by NAT_1:14;
then [1,j1] in Indices G1 by ;
then G1 * (1,j1) in { (G1 * (i,j)) where i, j is Nat : [i,j] in Indices G1 } ;
then G1 * (1,j1) in Values G1 by MATRIX_0:39;
then G1 * (1,j1) in Values G2 by A1;
then G1 * (1,j1) in { (G2 * (i,j)) where i, j is Nat : [i,j] in Indices G2 } by MATRIX_0:39;
then consider i, j being Nat such that
A9: G1 * (1,j1) = G2 * (i,j) and
A10: [i,j] in Indices G2 ;
A11: ( 1 <= j & j <= width G2 ) by ;
0 <> len G2 by MATRIX_0:def 10;
then A12: 1 <= len G2 by NAT_1:14;
then A13: (G2 * (1,j)) `1 = (G2 * (1,1)) `1 by
.= (G2 * (1,j2)) `1 by ;
A14: i <= len G2 by ;
1 <= i by ;
then 1 < i by ;
hence contradiction by A4, A8, A9, A14, A11, A13, GOBOARD5:3; :: thesis: verum