On an Object, container() is defined as
result = self.get(self.owningProperty())
where owningProperty() is defined as
result = self.allProperties->select(op| op.isComposite and self.get(op)
<> null)

If I read this correctly, the container of an object is the value of a
property on that object such that isComposite on the property is true
and the value of the property on the object is not null.

How is that not backwards? The value of an object's composite properties
are the objects it contains. Don't we want (op|
op.opposite.isComposite and self.get(op) <> null)?

see below

In the specification [The “lower” bound of a MultiplicityType to be “Unbounded.” [C-54]] [The “upper” bound of a MultiplicityType cannot be less than 1. [C-56]] and it should be [The “upper” bound of a MultiplicityType to be “Unbounded.” [C-54]] [The “lower” bound of a MultiplicityType cannot be less than 1. [C-56]]

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In ptc/04-10-05, section 9.1, the documentation for isSet() states that it "returns true if the value of the Property is different than the default value of that property. "
However later on in the section under Semantics, it states:

If a single-valued property has a default:

....

• If the value of that property is later explicitly set, even to the default value, isSet=true,

Thus there is an inconsistency as to the value if isSet() in the case where the property has been explicitly set to a value that happens to be the same as its default.

It’s important to note that Property::default is used only for initial assignment on element creation, not as the value returned in absence of an explicit value.

Section 9.1 of ptc/04-10-15 defines operation convertToString that takes an Object parameter and converts to a string representation for a supplied DataType.
However no exception is defined for when the supplied Object is an instance of a ModelElement and not a DataType.

see below