Good day,
To people well acquainted with XML schemas manipulation here,
how do you express relationships/references between elements ?

Take the following excerpt from one of the analysts around me:

~ <xs:attributeGroup name=3D"traveller-id-ag">
~   <xs:attribute name=3D"traveller-id" type=3D"string20" use=3D"required"/=
>
~ </xs:attributeGroup>
~
~ <xs:attributeGroup name=3D"ticket-id-ag">
~   <xs:attribute name=3D"ticket-id" type=3D"string20" use=3D"required"/>
~ </xs:attributeGroup>
~
~ <xs:attributeGroup name=3D"segment-id-ag">
~   <xs:attributeGroup ref=3D"product-id-ag"/>
~   <xs:attributeGroup ref=3D"traveller-id-ag"/>
~ </xs:attributeGroup>
~
~ <xs:complexType name=3D"traveller">
~   <xs:attributeGroup ref=3D"traveller-id-ag"/>
~   <xs:attributeGroup ref=3D"modification-info-ag"/>
~   <xs:sequence>
~     <xs:element name=3D"traveller-info" type=3D"traveller-info"/>
~     <xs:element name=3D"remark" type=3D"remark" minOccurs=3D... />
~     <xs:element name=3D"message" type=3D"message" minOccurs=3D... />
~   </xs:sequence>
~ </xs:complexType>
~
~ <xs:complexType name=3D"segment">
~   <xs:attributeGroup ref=3D"segment-id-ag"/>
~   <xs:sequence>
~     <xs:element name=3D"segment-info" type=3D"segment-info"/>
~     <xs:element name=3D"price" type=3D"price" minOccurs=3D"0"/>
~     <xs:element name=3D"ticket-ref" minOccurs=3D"0">
~       <xs:complexType>
~         <xs:attributeGroup ref=3D"ticket-id-ag"/>
~       </xs:complexType>
~     </xs:element>
~     <xs:element name=3D"duration" type=3D"duration" minOccurs=3D... />
~     <xs:element name=3D"length" type=3D"length" minOccurs=3D... />
~   </xs:sequence>
~ </xs:complexType>
~
~ <xs:complexType name=3D"ticket">
~   <xs:attributeGroup ref=3D"ticket-id-ag"/>
~   <xs:sequence>
~     <xs:element name=3D"price" type=3D"price" minOccurs=3D"0">
~     </xs:element>
~     [...]
~   </xs:sequence>
~ </xs:complexType>

You see the guy has first declared attribute groups to specify
ID's (simple or composite), that he later uses inside complex
types as real ID's/keys or as references either (which kind of
annoys me): in the complex type "ticket", the "ticket-id-ag"
is a *key*, a string that identifies uniquely a ticket; but
in "segment", it's a reference to the ticket defined below
(but to express the "ref" difference we have, he's put this
"ticket-id-ag" inside an element named "ticket-ref")...

Shouldn't it be more clear-cut and legible to do something
like (and get rid of these attribute groups as well maybe ?):
~ <xs:complexType name=3D"segment">
~   <xs:key name=3D"segment-id"/>
~   <xs:sequence>
~     <xs:element name=3D"segment-info" type=3D"segment-info"/>
~     <xs:element name=3D"price" type=3D"price" minOccurs=3D"0"/>
~     <xs:element name=3D"ticket-ref" minOccurs=3D"0">
~         <xs:keyref name=3D"ticket-id-ref" ref=3D"ticket-id"/>
~     </xs:element>
~   </xs:sequence>
~ </xs:complexType>
~
~ <xs:complexType name=3D"ticket">
~   <xs:key name=3D"ticket-id"/>
~   <xs:sequence>
~     <xs:element name=3D"price" type=3D"price" minOccurs=3D"0">
~     </xs:element>
~     [...]
~   </xs:sequence>
~ </xs:complexType>

or something similar (I'm not sure at all of the syntax
here, perhaps "segment" could be further simplified with:
~     <xs:keyref name=3D"ticket-ref" ref=3D"ticket-id" >
~     </xs:keyref>
with no xs:element surrounding it ?
I have no idea - sorry I'm not very familiar with XML
schemas...) ?

A detail that might have its importance: this comes from
XSD's used with JAXB to generate classes of our protocol...

Thanks a lot in advance for any help and advice about all
this stuff !
Regards,
S=E9bastien Rigaud