Allow definition of first message field by parameter value

[treiher]

Context and Problem Statement

With the introduction of parameterized messages (#609) there is no strict need for a definite first field anymore, as the first field can depend on the value of a message parameter. Currently, the model is checked to have exactly one link to the first message field.

Considered Options

O1 Allow definition of first message field by parameter value

+ More flexible

O2 Keep current restriction

+ No changes needed

Decision Outcome

O1

[treiher]

We decided to support multiple initial links to allow the first message field to be defined by parameter values (O1).