"Query prolog" what's that? (used in docs)

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"Query prolog" what's that? (used in docs)

In "Teradata XML Release 14.10" in couple of places there is such text:

Note: Namespaces declared in the prolog of the queries override any namespace declarations specified here.

(emphasis mine)

 

 

What is meant here by "prolog of the query"?

(pointing to docs would be very helpful; google doesn't help much here)