RNC Question | Element Dependency on Enumerated Attribute

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Hi,

So, I'm writing this internal xml schema to standardise a document 
creation process. What I'd like to do is (this is in RNC)

given this pattern  --

element field {
	attribute type ("text"|"select"),	
	attribute length? &
	element options {
		element option {
			attribute value,
			text
		}*
	}?
  	....
}

what i'd like to be able to do is make another element pattern 
dependant on the selected 'type', so if 'text' is selected, 'length' is 
required, if 'select' is selected, the 'options' attribute is required.

The elements share too many common attributes (not shown here) to 
consider them seperate entities, but have minor, yet important 
differences that would be elegant to handle the way I'd like ?

Is this possible or am I barking up the wrong tree ?

TIA
Oli

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