Derivation by restriction

What is the correct syntax for derivation from complexType/simpleContent
types?
For example, if I write

<complexType name="MeasureType">
	<simpleContent>
		<extension base="double">
			<attribute name="uom" type="token"
use="required"/>
		</extension>
	</simpleContent>
</complexType>

<complexType name="LengthType">
	<simpleContent>
		<restriction base="gml:MeasureType"/>
	</simpleContent>
</complexType>

1. Is the "double" simpleContent preserved or not?
2. Is there any way I can restrict the value of uom?

Simon Cox
______
Simon.Cox@csiro.au  CSIRO Exploration & Mining
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