
The function returns the sum, for each node in every argument node-set, of the result of converting the string-values of the node to a number.
If some arguments are not node-sets, they are converted to numbers first.

Note that this definition differs from XPATH 1.0 standard, where sum() function
must have exactly one argument of type node-set.
It is important that other XPATH processors may quietly ignore all arguments except the first
one, producing unexpected results.

Being called without arguments, sum() will return zero.
