
The function creates a temporary local variable, whise name is specified by
varname argument.
Then, for every item of source_set sequence it
calculates the value of mapping_expn expression
having set the created variable to that "current" item.
It returns the "flattened" sequence of values returned by
mapping_expn in the same order as they are calculated.
"Flattened" means that if mapping_expn
returns an sequence, items of this sequence will be added into the end of
resulting sequence, one by one, instead of adding one item of type "sequence".

In any case, temporary variable is destroyed on return.

This function is used in the implementation of
"FOR" 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.
