No, there's no such method. (Does any other language have such a thing? Perhaps some purely interpreted shell languages.) Whether or not a variable is declared is a compile-time condition entirely under the stylesheet author's control, which is a big difference from doc-available() which tests a run-time condition outside the stylesheet's control. (And different from function-available(), which tests a compile-time condition outside the programmer's control).

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Michael Kay
http://www.saxonica.com/
Author, XSLT 2.0 and XPath 2.0 Programmer\'s Reference