DOCBOOK: Where is the declaration that say that element set is the root

Hi !

	First thing, I want to say to you I am "newbie" with docbook.

	To help me understand where the links are made with the different elements
you found in this DTD, I need to know where is the declaration that say that
element set is the root element ?

Thank you !
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Beno褾 S憝igny, Revenu Qu嶵ec
DSP-SSPVAL, 5-1-7
(418) 652-5658 (7568)