// Handle methods that produce output should observe the following rules:
	//   - If r.Time is the zero time, ignore the time.
	//   - If r.PC is zero, ignore it.
	//   - Attr's values should be resolved.
	//   - If an Attr's key and value are both the zero value, ignore the Attr.
	//     This can be tested with attr.Equal(Attr{}).
	//   - If a group's key is empty, inline the group's Attrs.
	//   - If a group has no Attrs (even if it has a non-empty key),
	//     ignore it.