reserve i,i1,i2,i9,i19,j,j1,j2,j9,j19,k,k1,k2,l,m,n for Nat;
reserve r,s,r9,s9 for Real;
reserve D for non empty set, f for FinSequence of D;
reserve f for FinSequence of TOP-REAL 2, G for Go-board;

theorem Th2:
  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
proof
  let G1,G2 be Go-board such 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);
  set p = G1*(1,j1);
A5: 1 <= j1 & j1 <= width G1 by A2,MATRIX_0:32;
  assume
A6: i1 <> 1;
  1 <= i1 by A2,MATRIX_0:32;
  then
A7: 1 < i1 by A6,XXREAL_0:1;
  i1 <= len G1 by A2,MATRIX_0:32;
  then
A8: p`1 < G1*(i1,j1)`1 by A5,A7,GOBOARD5:3;
  0 <> len G1 by MATRIX_0:def 10;
  then 1 <= len G1 by NAT_1:14;
  then [1,j1] in Indices G1 by A5,MATRIX_0:30;
  then p in {G1*(i,j): [i,j] in Indices G1};
  then p in Values G1 by MATRIX_0:39;
  then p in Values G2 by A1;
  then p in {G2*(i,j): [i,j] in Indices G2} by MATRIX_0:39;
  then consider i,j such that
A9: p = G2*(i,j) and
A10: [i,j] in Indices G2;
A11: 1 <= j & j <= width G2 by A10,MATRIX_0:32;
  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 A11,GOBOARD5:2
    .= G2*(1,j2)`1 by A3,A12,GOBOARD5:2;
A14: i <= len G2 by A10,MATRIX_0:32;
  1 <= i by A10,MATRIX_0:32;
  then 1 < i by A4,A8,A9,A13,XXREAL_0:1;
  hence contradiction by A4,A8,A9,A14,A11,A13,GOBOARD5:3;
end;
