While the examples above use parantheses, one of the benefits of Polish notation is that, assuming the arity of each operator is known, parentheses are unnecessary: the order of operations is unique and easy to determine, if the expression is known to be correct. For example, assuming * and + are binary,
This notation is described briefly in section 6 of the spec.
There is also a rarely-used declaration called a notation, which creates a name-location pair that can be used to signal to an application that a parsed character data section is to be interpreted as having some kind of additional encoding, such as Base64 or PostScript.
The "xml:" prefix does not have to be declared in an xmlns:xml attribute, though; the XML Namespaces recommendation says that xml: is by default bound to a particular namespace.