The function pool is generic. Its purpose is to combine
data from several objects of the same type (typically computed
from different datasets) into a common, pooled estimate.
The function pool.anyist is the method
for the class "anylist". It is used when the objects to be
pooled are given in a list x.
Each of the elements of the list x, and each of the
subsequent arguments ... if provided, must be an object of the same
class.