UML14 — UML 2 Super/Interactions/Need unattached lifelines

At present, a lifeline always requires a corresponding ConnectableElement. For informal users of UML this requires that have to declare a specific structure for every interaction diagram that they want to draw. However, there are many uses of UML who want a less formal approach. For example, people may want to attach scenarios to use cases informally, that is, without having to talk about any specific connectable elements.

Therefore, the multiplicity of the Lifeline::represents association end should be changed from 1 to 0..1.

see above