QVT11 — Inconsistent Rule.transformation multiplicity/composes for Mapping

Mapping is a Rule and Rule.transformation has unit multiplicity and is a container.
Therefore Rule.context can never be a container.

This could be fixed in a number of ways:

Fix 1: Change Rule.transformation to 0..1
--> allow hierarchical Rule containment
--> all Transformation rules are distinctly named
– no representation change
– minor semantic relaxation for QVTr/QVTo
– minor semantic surprise for QVTc - composed mapping has null Mapping.transformation.

Fix 2: Change Rule.context to not composes
--> require flat Rule containment
--> Transformation can have multiple unnamed rules
– no change for QVTc/QVTo
– representation change for QVTc

Fix 3: Change opposite of Transformation.rule from Rule.transformation[1] to Rule.context[1]
Redefine Rule.transformation[1] as a derived property
Remove mapping.context (inherited from Rule)
– Doesn't work: context needs to be NamedElement to be either Rule or Transformation

Fix 4: Change opposite of Transformation.rule from Rule.transformation[1] to Rule.owningTransformation[0..1]
Redefine Rule.transformation[1] as a derived property
Rule.transformation := owningTransformation
Mapping.transformation := if context then context.transformation else owningTransformation endif
– no representation change
– minor semantic relaxation for QVTr/QVTo

Recommend 4.

Rule.transformation changed to 0..1 in QVTCore.xml for QVT 1.1.
Fix 4 doesn't seem that wonderful, so go with the obvious Fix 1. And why prohibit all reuse of Rule
outside a Transformation?