theorem Th18:
  G is X_equal-in-line implies v_strip(G,0) = { |[r,s]| : r <= G*( 1,1)`1 }
proof
  0 <> width G by MATRIX_0:def 10;
  then
A1: 1 <= width G by NAT_1:14;
  assume G is X_equal-in-line;
  hence thesis by A1,GOBOARD5:10;
end;
