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It is stated in section 5.7.1 p:variable that: "The variable's value is specified with a select attribute. The select attribute must be specified. The content of the select attribute is an XPath expression which will be evaluated to provide the value of the variable." The section then goes with statements such as "If a select expression is given...". Since the select attribute is mandatory, the select expression must necessarily have been given.
Agreed. The text that suggests that the expression may not be given is an editorial error.
Thanks.