UML22 — "PackageableElement::visibility" uses "false" as default value

In the Superstructure spec 2.1.1, the description of
"PackageableElement::visibility" says: "Default value is false."
However, "false" is not a valid value for the type "VisibilityKind".

I suggest that the default visibility be defined as "public". While
it may make sense for properties in a class to be private by default,
this is not the case for packageable elements, here it makes way more
sense to have a public default. The description should be changed
accordingly.

Second, the UML Metamodel CMOF files define metaclass
"PackageableElement" to be a specialization of metaclass
"NamedElement" without redefining "visibility". However, the
metaclass description in the superstructure spec does redefine
"visibility". The CMOF files should be adjusted to make the same
redefinitions the description makes.

Regarding the first point,.PackageableElement::visibility already has „public? as a default value in the normative CMOF models, so the description in the spec text needs to be corrected as suggested in the issue.
Regarding the second point, PackageableElement::visibility in the CMOF models already redefines NamedElement::visibility, matching the spec doc, so no change is needed.