UMLR — Semantics of Executable Nodes does not cover Control Flows on Control Pins

15.5.3 Semantics
The Executable Nodes discussion requires all ControlFlows to be implicitly joined (effectively made mandatory). It should be possible to have optional ControlFlows if they arrive on pins (e.g., as part of a ParameterSet). It may also be possible to have optional ControlFlows if the implicit join could have a joinSpec by the use of constraint.
Similarly, it should be possible to use a ParameterSet to have some controlPins w/o output control tokens.