UMLR — Semantics of Lifeline.selector not clear

In UML 2.4.1 the semantics of Lifeline.selector is defined as "If the referenced ConnectableElement is multivalued (i.e, has a multiplicity > 1), then the Lifeline may have an expression (the ‘selector’) that specifies which particular part is represented by this Lifeline."

This part (even though not very precise) is completely removed from UML 2.5, section 17.3.3. Instead a constraint has been introduced that restricts the selector ValueSpecification to being LiteralString or LiteralInteger, without further explaining how the corresponding parts out of a multivalued part are selected.

Since parts (i.e., metaclass Property) may represent unordered collections, the selector should rather be restricted to evaluate to a Boolean expression. The Lifeline would represent select all instances contained in the multivalued part for which the Boolean expression evaluates to true.

No technical changes to the metamodel required, but editorial changes and update of Constraints.