The Namespaces container is a read-only list
used to hold the additional namespace declarations of an
element. It provides indexed access for convenience,
but the order is neither predictable nor reproducible,
and has no meaning.
Return the URI associated with a prefix, as determined
by the namespaces stored in this list. This method
returns null if the prefix is not found in the list.
Parameters: prefix - the prefix whose URI is desired the namespace URI for this prefix, or null if this prefix is not not mapped to a URI by these namespace declarations