Relax NG external reference question

[Ken]

For illustration I have three documents

1) A document of definitions
<options>
<option1>
<term>X</term>
<definiton>THE LETTER X</definition>
</option1>
<option2>
<term>Y</term>
<definiton>THE LETTER Y</definition>
</option1>
<option3>
<term>Z</term>
<definiton>THE LETTER Z</definition>
</option1>
</options>

2) An document that uses the terms where a particular tag may only
have one of the elements that appear in the <term> tag from the first
document.

3) A Relax NG document that enforces the structure of document 2...

I am new to Relax NG I know about the choice tag in Relax NG but not
how to reference the elements of the term tag from document 1 as
values for the choice.