Provides information about a function which is helpfull at design time
like following:
+-- function name
+-- short description
+-- long description
+-- is operation or function
+-- supported types of arguments
+-- type of result
+-- quantity of arguments
+-- priority level (for operations only).
Calculates the result type of a function (operation) by the list
of parameters' types.
The implementaion class can also perform type validation for parameters.
This extension function is used as special stub to generate a valid XPath
expression in case when required arguments of operation or function is skipped.