HowTo describe some things using XML Schema

hi

im was develop an own format of XML document and want to describe this  
format using XML Schema.

for example i have a ComplexType called "Cloud" with next schema:
======================================================================================
<xsd:complexType name="Cloud">
	<xsd:sequence minOccurs="0" maxOccurs="1">
		<xsd:element name="parentclouds" minOccurs="0" maxOccurs="1">
			<xsd:complexType>
				<xsd:choice minOccurs="0" maxOccurs="unbounded">
					<xsd:element name="cloud" type="CloudObject"/>
				</xsd:choice>
			</xsd:complexType>
		</xsd:element>
		<xsd:group ref="MandatoryAttributes" minOccurs="0" maxOccurs="1"/>
		<xsd:group ref="Attributes" minOccurs="0" maxOccurs="unbounded"/>
	</xsd:sequence>
	<xsd:attribute name="id" type="xsd:NMTOKEN" use="optional"/>
	<xsd:attribute name="name" type="xsd:string" use="required"/>
	<xsd:attribute name="type" type="xsd:string" use="optional"/>
</xsd:complexType>
======================================================================================

1. How to describe a Key which can be declared using @id attribute in any  
elements of "Cloud" type.
2. How to describe a KeyRef which can refers to the @id attribute (to the  
key from first question)
3. How to describe an element which is either a "Cloud" or it can be  
simple element with one attribute called @refid which is a KeyRef from  
second question..

More simple.. how to describe this document:
<cloud id="123" name="newcl"/>
<cloud refid="123"/>
<cloud id="1234" name="test"/>
<cloud id="1234" name="test2"/> <-- an error key not unique
<cloud refid="xxx"/> <-- an error key not declared
<cloud refid="1234" name="test2"/> <-- an error (???), refid is used...

i.e.: when using @refid - the @name and @id attributes must be prohibited  
and vice-versa

TIA

-- 
...with best regards && have a nice day...