code: func TOS(tos uint8, label uint32) NetworkChecker {
  	return func(t *testing.T, h []header.Network) {
  		t.Helper()
  
  		if v, l := h[0].TOS(); v != tos || l != label {
  			t.Errorf("Bad TOS, got (%v, %v), want (%v,%v)", v, l, tos, label)
  		}
  	}
  }
docs: // TOS creates a checker that checks the TOS field.