Single-pass mean and error calculation

Here is a snipped for a function that computes a mean and standard error in the mean (SEM) in a single pass:

from npstreams import imean, isem, array_stream, itercopy

# The `array_stream` decorator ensures that the elements of
# the iterable `arrays` will be converted to ndarrays if possible
# This decorator is not required.
def mean_and_error(arrays, axis = -1):
    """ Yields (mean, error) pairs from a stream of arrays """
    # itercopy creates a copy of the original stream
    # The elements are only generated once, and then fed
    # to those two copies; much more efficient than
    # creating two streams from scratch.
    arrays_for_mean, arrays_for_sem = itercopy(arrays)

    means = imean(arrays_for_mean, axis = axis)
    errors = isem(arrays_for_sem, axis = axis)

    yield from zip(means, errors)