An edge may not leave a hidden output port
The rule
Normative: this is the rule
An edge whose source port is declared by the source node's node type but is not exposed on that node is refused with R7_EDGE_SOURCE_NOT_EXPOSED at edge.{index}.
Rule identifiers are permanent and are never renumbered. Each implementation publishes its own standing against these rules; this specification does not.spec 1.0-draft · R7.b · changed in spec 1.0