Validity errors

Well-formed XML documents have an infoset even if they are invalid.
But some kinds of invalidity make it hard to assign reasonable values
to certain properties.  For example, a duplicate ID makes it
impossible to assign a meaningful value to an the [references]
property of an IDREF attribute referring to that ID.  This particular
case is handled in the spec - the [references] property has no value -
but there are others that aren't, such as the value of the [notations]
property when there are multiple declarations of the same notation
name.

We should find all such cases and resolve them, or perhaps state a
general rule.

-- Richard