serialization ambiguity between features with a single value

( value+=[abs::Asset|QualifiedName] |
	'[' value+=[abs::Asset|QualifiedName] (',' value+=[abs::Asset|QualifiedName])* ']'
)

(and similar in attributes)

Leads to an ambiguity between collection with a single element, and simple values.

Fix: check at serialization time the multiplicity of the feature.