let i1, i2, j1 be Nat; :: thesis: for G1, G2 being Go-board st Values G1 c= Values G2 & [i1,j1] in Indices G1 & 1 <= i2 & i2 <= len G2 & G1 * (i1,j1) = G2 * (i2,1) holds
j1 = 1

let G1, G2 be Go-board; :: thesis: ( Values G1 c= Values G2 & [i1,j1] in Indices G1 & 1 <= i2 & i2 <= len G2 & G1 * (i1,j1) = G2 * (i2,1) implies j1 = 1 )
assume that
A1: Values G1 c= Values G2 and
A2: [i1,j1] in Indices G1 and
A3: ( 1 <= i2 & i2 <= len G2 ) and
A4: G1 * (i1,j1) = G2 * (i2,1) ; :: thesis: j1 = 1
set p = G1 * (i1,1);
A5: ( 1 <= i1 & i1 <= len G1 ) by ;
assume A6: j1 <> 1 ; :: thesis: contradiction
1 <= j1 by ;
then A7: 1 < j1 by ;
j1 <= width G1 by ;
then A8: (G1 * (i1,1)) `2 < (G1 * (i1,j1)) `2 by ;
0 <> width G1 by MATRIX_0:def 10;
then 1 <= width G1 by NAT_1:14;
then [i1,1] in Indices G1 by ;
then G1 * (i1,1) in { (G1 * (i,j)) where i, j is Nat : [i,j] in Indices G1 } ;
then G1 * (i1,1) in Values G1 by MATRIX_0:39;
then G1 * (i1,1) in Values G2 by A1;
then G1 * (i1,1) 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 * (i1,1) = G2 * (i,j) and
A10: [i,j] in Indices G2 ;
A11: ( 1 <= i & i <= len G2 ) by ;
0 <> width G2 by MATRIX_0:def 10;
then A12: 1 <= width G2 by NAT_1:14;
then A13: (G2 * (i,1)) `2 = (G2 * (1,1)) `2 by
.= (G2 * (i2,1)) `2 by ;
A14: j <= width G2 by ;
1 <= j by ;
then 1 < j by ;
hence contradiction by A4, A8, A9, A11, A14, A13, GOBOARD5:4; :: thesis: verum