The function returns the average of all values in all its
arguments, For each node in every argument node-set, it converts
the string-value of the node to a number and adds the result to the
sum. If some arguments are not node-sets, they are converted to
numbers first and added to the sum. Then sum is divided by number
of values added and the result is returned.
This function is not a part of XPATH 1.0 standard library.
A node-set of nodes whose string values must be converted to
numbers and added to the result, or single value.
Nonempty sequence of values expected as
argument(s) of XPATH function avg(); avg() of nothing is
This happens if the function is called without arguments, or
with all arguments set to empty node-sets.