Allow segments within segment definitions

Use case: I create a segment for those who joined last month. Now, I would like to add an additional segment that includes a subset of this segment. Instead of recreating the original segment in the new one (and needing to update both in the future should they need to change), it would be fantastic to use one segment as a filter starting point. I have used a simplified example above, but this can easily get more complex (and more annoying to maintain) over time.