SYSML21 — Succession to start and succession from done should be invalid

Currently, specifying a succession from a subaction to the start snapshot is allowed by the specification, even though this is not really meaningful or useful.

Consider this example model:

action a0

Action a1 is a composite subaction of a0, so its performance must be within the lifetime of a performance of a0. Since "start" is the first snapshot of the performance of a0, nothing can happen before the "start" within the performance (lifetime) of a0. Therefore, the only legal performances of a0 are ones in which a1 is not performed. On the other hand, if you assign a multiplicity of [1..1] on a1, then a0 has no valid performances.

This is not a satisfiable model, so at a minimum, modeling tools should flag this as a warning, or better yet, as an error.

A similar argument can be made about a succession from "done" to another subaction, such as in the following model:

action a0