Chameleon schemas <include>ing chameleon schemas

I frequently come across the case where a schema document with no
targetNamespace attribute (call this one simpleTypes.xsd) is
<include>d in another schema document with no targetNamespace
attribute (say, contactTypes.xsd), which is, in turn, <include>d in a
schema document with a targetNamespace attribute (say, main.xsd having
a targetNamespace attribute value of "apd"). (I am using "apd" as a
URI here to save typing.)

Clearly, any data type defined in contactTypes.xsd will exist in the
"apd" namespace. What about data types defined in simpleTypes.xsd? Do
they exist in the "apd" namespace, or do they have no target
namespace?

At least two suppliers of XML tools have different interpretations of
this. My solution is therefore to avoid the problem by making target
namespaces explicit, but my enquiring mind would love a definitive
statement on which interpretation is correct. Perhaps the tool vendors
will then take note.

I hope this is clear. I have examples if anyone wants them.

Regards

Paul Spencer