We currently have an arrow syntax for specifying dependency edges (Res[foo] -> Res[bar]), and a function for making containment edges, but not a syntax for making containment edges.
The function only works to add a specified resource to the current container (e.g., to specify that a resource is contained by a class), which works fine to remove the need for the 'anchor' pattern, but it doesn't work to specify containment outside of a container.
This would be useful for things that behave like containers in real life but aren't containers in Puppet's language, like virtual machines.
The arrow that seems obvious to me is '>>', because (AFAIK) it's not used and it has the right implication, but I'm open to other ideas.
I'm assuming this is quite simple to implement, given we've got equivalent syntactical structures already, and equivalent semantics, so while this isn't necessarily super-high priority, I'd like to see it done.