theorem Th21:
  G is Y_equal-in-column implies h_strip(G,0) = { |[r,s]| : s <= G *(1,1)`2 }
proof
  0 <> len G by MATRIX_0:def 10;
  then
A1: 1 <= len G by NAT_1:14;
  assume G is Y_equal-in-column;
  hence thesis by A1,GOBOARD5:7;
end;
