'Creates a new L{ExponentiallyDecayingSample}.
:type size: C{int}
:param size: the number of samples to keep in the sampling reservoir
:type alpha: C{float}
:param alpha: the exponential decay factor; the higher this is, the more
biased the sample will be towards newer values
:type clock: C{function}
:param clock: the function used to return the current time, default to
seconds since the epoch; to be used with other time
units, or with the twisted clock for our testing purposes'

def __init__(self, size=DEFAULT_SIZE, alpha=DEFAULT_ALPHA, clock=time):

    super(ExpDecayingSample, self).__init__()
    self.clock = clock
    self.size = size
    self.alpha = alpha
    self.clear()
