QVT14 — QVTr : Missing multi-when/where invocation

In QVTo the a paremnt mapping may pull its child mappings by e.g.

result.children := self.children->map Child2Child();

QVTr has no counterpart to this. It is needed by those pursuing a procedural style with where clause sequencing. Relations such as RVarSetToMBVarSet in RelToCore demonstrate the need for a multi-invocation.

Since multi-directional usage prohibits the use of a result as in QVTo, suggest that a 1:1 relation invocation may be invoked to perform a N:N mapping by passing a collection where a non-collection was expected to at most one of the root variables of each domain.