code: func FragmentFlags(flags uint8) NetworkChecker {
  	return func(t *testing.T, h []header.Network) {
  		t.Helper()
  
  		// We only do this of IPv4 for now.
  		switch ip := h[0].(type) {
  		case header.IPv4:
  			if v := ip.Flags(); v != flags {
  				t.Errorf("Bad fragment offset, got %v, want %v", v, flags)
  			}
  		}
  	}
  }
docs: // FragmentFlags creates a checker that checks the fragment flags field.