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In Section 5.7.2 p:option of the XProc specification, it is stated: "Regardless of the implicit type of the expression, when XPath 1.0 is being used, the string value of the expression becomes the value of the option; when XPath 2.0 is being used, the value is an xs:untypedAtomic." The specification fails to describe how a sequence of more than one item is converted to a string value. The XProc specification appears not to define the string value of an expression. XPath 2.0 defines "string value" in terms of a single node.
We believe the right answer is to adopt the XSLT rules for constructing simple content: 1. Zero-length text nodes in the sequence are discarded. 2. Adjacent text nodes in the sequence are merged into a single text node. 3. The sequence is atomized. 4. Every value in the atomized sequence is cast to a string. 5. The strings within the resulting sequence are concatenated, with a space separator inserted between successive strings. On the one hand, given that it's not specified, perhaps it should be changed to implementation defined, but that seems highly non-interoperable.