[Component] In scope functions [Value] 
The core functions defined in [Functions and Operators], together with the functions element-available, function-available, type-available, and system-property defined in this specification, plus the set of extension functions that are present in the static context of every XPath expression (other than a use-when expression) within the content of the element that is the parent of the use-when attribute. Note that stylesheet functions are not included in the context, which means that the function function-available will return false in respect of such functions. The effect of this rule is to ensure that function-available returns true in respect of functions that can be called within the scope of the use-when attribute. It also has the effect that these extensions functions will be recognized within the use-when attribute itself; however, the fact that a function is available in this sense gives no guarantee that a call on the function will succeed.