tan:data-type-check($item as item()?, $data-type as xs:string) as xs:boolean
Input: an item and a string naming a data type
Output: a boolean indicating whether the item can be cast into that data type
If the first parameter doesn't match a data type, the function returns false
Used by template ŧ core-expansion-terse
Does not rely upon global variables, keys, functions, or templates.