SYSML2 — Textual notation for send actions is too limited

According to the SysML Specification, 8.2.2.16.4 Send and Accept Actions, the general textual notation for a send action has the form

send payloadExpression via senderExpression to receiverExpression

In this syntax, the via and to parts are optional, but the payloadExpression is not. Since the result of the required payloadExpression is bound to the payload parameter of the send action, this makes it difficult to model a case in which, e.g., it is desired use a flow to provide the value of the payload, rather than the result of an expression. It also makes it difficult to model a general send action with the payload not specified (or defaulted), with the actual payload bound in a specialization of the general action.

While it is possible to get around these limitations by modeling the send action using a regular action usage typed by Actions::SendAction, this is no longer syntactically a send action usage and so, e.g., would not be rendered as such in the graphical notation. It would be better if the textual notation allowed the payloadExpression to be optional, while still declaring syntactic send action usage.