
This function calculates the value of test argument.
If the value is true, the function calculates the then_branch
expression and returns its value.
If the value is false, the function calculates the else_branch
expression and returns its value.

Note that unlike other programming languages, else_branch
is required argument, not optional.

This function is used in the implementation of
"IF" control operator in XQUERY,
so you will probably use that operator in XQUERY expessions, not the function.
This function may be useful in XPATH expressions and in XSLT stylesheets.
It is not a part of library of standard XQUERY 1.0 functions.
