Channel with 1 end is a legal syntax in cluster

[mypsycho]

Usually connection have at least 2 ends. Currently single port is allowed:

Channel: {Channel}
    ( annotations+=Annotation )*
    '{' (
        ( instancePorts+=InstancePort (',' instancePorts+=InstancePort)*
            ( ','   externalPort=[Port|IDENTIFIER] (',' instancePorts+=InstancePort)*)? )
    |
        ( externalPort=[Port|IDENTIFIER] (',' instancePorts+=InstancePort)* )
    )? '}'
;

Should be (TBC) ?

Channel: {Channel}
    ( annotations+=Annotation )*
    '{' ( externalPort=[Port|IDENTIFIER] (',' instancePorts+=InstancePort)+ ) // external port first
        | ( instancePorts+=InstancePort                                       // external port then
                    (',' instancePorts+=InstancePort)* 
                    ',' externalPort=[Port|IDENTIFIER] 
                    (',' instancePorts+=InstancePort)* )
           | ( instancePorts+=InstancePort (',' instancePorts+=InstancePort)+ )   // no external port
         )  '}'
;

[mypsycho]

Low priority: There is not user-impact for well-formed models. It may even be for compliant with language definition.

[mypsycho]

This rule is implemented in custom validation rule. No design rationale is identified for such choices. There is no reason to fix this element prior any others in XText.