reserve n,i,j,k for Nat;

theorem Th1: :: GR_CY 10
  k in SegM n iff k <= n
proof
  thus k in SegM n implies k <= n
  proof
    assume k in SegM n;
    then ex i being Nat st k=i & i <= n;
    hence thesis;
  end;
  thus thesis;
end;
