A proto node is an intermediate step between the `DocumentNode` and the boxed struct that actually runs the node (found in the [`BorrowTree`]). At different stages in the compilation process, this struct will be transformed into a reduced (more restricted) form acting as a subset of its original form, but that restricted form is still valid in the earlier stage in the compilation process before it
source not stored for this graph (policy: none)
no outgoing calls